I asked this question a little over a year ago, and, at the time, overwhelmed by the multitude of responses (and the nerve-racking decision ahead of me), I didn't quite follow up as well as I would have liked to. Which is a shame, because these answers and comments gave me the caution to guard myself and my coworkers from an undesirable or awkward parting, but at the same time sufficient courage to take into account my unique relationships in my company and do what I felt was right for me, for a wonderful result in the end. I can't thank everyone enough for their insights and anecdotes. In the end I took a "compromise" between the various answers, as well as my own thoughts, so I was hesitant to accept any one answer as correct.
In light of Workplace.SE graduating from beta, and today marking exactly one year since my move to the West Coast, here's an update!
tl;dr — I gave 1 months' notice, and offered to stay up to 2 months.
Through the December I asked this question, I was content to sit back, review answers and comments in this thread, and mull it over and wait, as my managers and coworkers were in and out of the office for the holidays anyway. As the new year rolled past and everyone got back into their routines, I asked my manager if I could speak with him in a conference room.1 I wasn't sure whether to include my team-lead in the same meeting—it seemed somewhat disrespectul not to—but I decided I'd simply make it a point to ask my manager about it first thing during the meeting, and call in the team-lead if it was a good idea to do so.2 I broke the news to my manager, saying it wasn't job-related or anything personal against coworkers or the company, but just a desire to move to the West Coast, and that I was committed. As I had a comfortable relationship with my manager, I elaborated a bit on why I wanted this move for myself (I suppose also to relieve my own guilt and reassure him that it wasn't anything job-related; but he'd have asked out of curiosity anyway). Of course he wasn't ecstatic, but all things considered he reacted quite optimistically, and clearly not upset with me, which is what I had feared (though in retrospect I had no reason to). The next thing that happened completely took me by surprise, though it was mentioned by several users.
Here, I think a disclaimer (and simultaneously my own "answer" to this question) is warranted:
Things worked out for me, but, as the diversity of answers in this thread already hint, your answer really depends on your circumstances, the leadership and culture in your company, as well as your values and priorities. My particular "success story" should not inspire anyone to follow suit; I'm just contributing one more case study for readers to consider and factor into their own assessments. Meanwhile, to answer my own question, i.e. whether it's "naive to tell my current employer that I'll be leaving, months in advance," succinctly, I'd say: Yes, it is naive to take such risks, especially unprepared (e.g. 3 months without salary), and naive to assume a company wouldn't protect its own interests first. But, jobs and companies do not exist in a vacuum, and if in your own assessment of your circumstances (e.g. an understaffed team), your work quality, and your work relationships (keeping in mind that this is a self-assessment and may not accurately reflect reality—i.e. are you really as valuable as you think you are?), you earnestly believe that an extended notice would be considerate, appropriate, and beneficial to all (and you're prepared for the worst case, to avoid being naive ;-) then that's an assessment no one else can make for you: I'd say, after you've covered all the bases, do what you feel is right, and won't regret.
I cut down the whopping 3 months' notice I'd intended to give, to 4 weeks, as had been suggested by some. But, I offered to stay up to 8 weeks—making it an option for them, and an ideal case for me. My manager, after hearing my case, said that for me, though he emphasized not for most others, the company might be willing to set up a remote arrangement, on a trial basis. I was stunned, since at best I was expecting to be allowed to work another 8 weeks. To this day I don't feel deserving of such consideration, and am grateful that my manager held me in such regard. He subsequently consulted upper management, I got the thumbs up the next day, and within a week HR summoned me to sign a remote working agreement. (It's my understanding that only one other employee in the office had been granted this privilege before, due to special circumstances; fortunately for me, this meant a template for a remote working agreement had already been drafted up and available for me.)3 The agreement was that my salary would be cut by a certain percentage (for no longer having to commute, maintain business casual clothing, or pay other New York City-related costs), that I'd continue working East Coast hours (5:30am to 2:00pm local time), and that there'd be an evaluation in 6 months to see whether all this is working out. They made it very clear that all this was a "trial period," not a guarantee of employment beyond the period; I'd be swiftly let go if things didn't work out. All that said and done, 6 weeks later (1 year ago today), I was on a flight to California.
1 I imagine most companies do this, but I want to be clear about the context for others who may not have this luxury in their workplace. My company requires managers to speak to employees twice a year, personally (one-on-one) and explicitly, that they should feel comfortable requesting additional one-to-one meetings, at any time of the year (not just during annual performance evaluations), about anything. Despite the obligatory nature, for me at least, it did make me feel very comfortable asking my manager for such a meeting.
2 It turned out that speaking to my manager privately, first, was the right thing to do, which, in retrospect, seems obvious for numerous reasons. Not at all that a team-lead doesn't deserve as much respect or visibility, but that it's the manager's role to handle these types of situations, and also the manager is privy to more information (e.g. budgets, my salary, staffing prospects, etc.) relevant to processing such news.
3 I am being careful throughout not to reveal information my employer would not want revealed. In sentences such as these I'm more trying to emphasize the rarity of this action by my employer so as to discourage others who may be familiar with my employer, from drawing any conclusions.