The more dense and urban an area is seems to have a lot to do with cyclists' behavior on this. In Chicago, I will certainly split lanes when I feel it's safe, and really, especially when going between meetings downtown during the day, if not for splitting lanes I may as well walk or ride the train - those two options would probably be faster.

I did, last year, have a little accident though - after traffic started at a green light, I cut into a space between two cars in the right lane of the two I was splitting (after signaling my intentions and making eye contact with the car behind me). Turned my head back around to find that the car in front had slowed down significantly - I'm sure it was an extremely important text or Facebook status update. I ended up hitting the back bumper with my front wheel, and laying the bike down in the parking lane - scraped up my knee and elbow and busted my front derailer, nothing worse than that. I've been much more careful since then about lane-splitting and trying to think a few more steps ahead - I also make it a point to steer clear of trucks and buses, both because I don't want to die, and because, really, the thirty people on that bus can totally have the right of way versus the one of me on the bike.

All that said, if I'm not in a rush, I don't mind waiting in a line either.