First some context: I graduated college 3 months ago near the top of my class in Computer Science. Instead of going on to work at Google or Facebook or some similarly interesting company (like many other top performers in my class), I decided to take the first opportunity that came to me and work locally. I had a lot of reasons for this - no travel, burnt out on 60+ hour weeks, really needing a job out of college, etc. The job did not sound difficult, but I wanted a job and I was assured that the work I'd be doing was "more difficult than it sounds."
The first day I arrived at work, I was told what I'd be doing for the duration of my employment and frankly, it turned to be exactly as (non)difficult as it originally sounded.
Having been there three months, I can say my time was spent in a 70-30 split. 30% of the time, I did painfully trivial work. For all intents and purposes, their idea of programming amounted to filling out x and y values on glorified CSS sheets. The other 70% - and the part that really gets to me - was spent doing absolutely nothing. For the first few weeks, I would make rounds to my supervisor and the other developers asking - eventually even begging - for some task to do. The answer was always "sorry nothing yet, hang in there." Eventually I gave up on that, as it was clearly going nowhere.
Now, after accepting a much more interesting and (hopefully) fulfilling job elsewhere, I gave my 2 weeks notice, because I believed it was appropriate to do so. I mentioned that if they did not need me to stay for 2 weeks, I was okay with leaving early, though internally I was hoping they would simply let me go. That was on Monday, and they have not made any effort to take it further. In fact, they've done just as little acknowledging of my existence as they've done the entire time I've been here. All my files are in their git repo - in practice all I need to do to leave is hand in my key.
So, my question is, given that I'm doing exactly no work right now (and no one has any desire to give me any), is it inappropriate for me to tell them tomorrow that I've changed my mind and will be leaving immediately? If so, what's a good way to say it? I don't want to insult them, but I will not be putting my time here on my resume, nor will I be using them as a reference (how could I? I've done no work).
Please note that I can start my new job a week early if I want to, so time off isn't an issue. My main motive for wanting to leave early is simply that it borders on physically painful to be forced to waste such inordinate amounts of time.
Update: I spoke with my boss today, and basically said something to the effect of "I have no tasks, is it really necessary that I stay?" He seemed surprisingly fine with letting me go, but told me to talk to his boss, which I did. His boss did not seem too thrilled about the idea of me leaving, but there's a chance that he might go for it. I'll have to see what happens.
I also feel inclined to respond to everyone's different opinions. Hopefully the way I handled the situation was professional enough; it may not have been but that's in the past now. In response to the assertion that I should have used to the time to develop new skills: I did. I learned quite a lot while here, no thanks to any tasks they gave me to do, but there's still two issues I had with using this time. First, I could not make anything and therefore I still was not producing anything valuable to my resume, and whatever I made still would not have looked as good as true professional experience. Second, after a while (especially now), that has become difficult to do. Not having the freedom to actually code anything makes learning about a language progressively more difficult (and less fun).
As to why I took the job... there really isn't a good reason. This isn't a start-up, it's a moderate size company that does work so boring that when I explained what the company did on interviews (not even what I did) some interviewers laughed and didn't even bother to ask why I wanted to leave.
It's hard to explain exactly why I took the job - I asked myself that quite a bit - and I think the simple answer is that the offer came at I time when I was really burnt out on school, and I really just wanted something easy. In fact, I got the offer before I had really done any job hunting. On one hand, I could have started off somewhere better to begin with if I had waited, but at least by taking this job I had income while looking, so I don't think I actually regret taking it.
So, in conclusion, thank you everyone for the help.