Reviewing ahead of time
There's not much ettiquette out there - the medium is just too new. I'd say it's still widely variable, but most people treat their profiles as an extension of their public identity - a business card + a resume.
I'd say there's not any harm in looking, and that it's quite unlikely that the interviewer will even notice. Never say never, but if you have a busy profile, it's unlikely you'll take the time to check.
Keep the focus on the work, though. Learn about the job the interviewer does at the company you'll be interviewing in, and move on to what the company does. If you happen to notice a 1st degree connection, it's OK to make note of it and mention it in the interview - but don't stretch for it. Noticing that you share a connection, went to the same school, worked at the same company, or are part of the same volunteer organization is fine. Figuring out the 7 degrees that separate you and having them memorized... is creepy.
One exception: If you happen to realize that the interviewer is someone who has a very public face, who has contributed information/ideas that you admire - it's OK to say it. As a person who teaches, lectures, and writes (here!) alot - I've never minded it, in fact it's nice to know where the market penetration is. Again, though, stay on the casual side "hey, I liked your article/speech/class on Y", and not a long gushing statement about your admiration.
Companies vs. People
You can get a lot more in depth with the company itself, and that's a plus, not a minus. After checking out the profile, do what you can to dig around Linked In or any other source, and see what the company is doing. Knowing something about their business, their suppliers, their current intiatives - is all good stuff. If it comes from LinkedIn - that's fine.
Things not to do with Linked In
The things to avoid are generally the actions - suggesting a connection before you've met is the big one. You're not connected, you don't know each other.
If you get along in the interview and it feels casual, it's OK to ask at the end of the interview - 'Hey, it was great meeting you - mind if I connect to you on LinkedIn?" Do this with a certain caution - there are companies with policies about this, and you may get the response that he simply can't. And don't do it if it isn't genuine - it'll feel false.
And don't use it as a back channel to get around the formal interview process - if you have an official contact relating to interview logistics, stick to them.