I've been using svn for all my papers for about a year, and I can't imagine not using some kind of version control now. I've had no problem with getting coauthors to use it, though I don't have any technophobe collaborators.
The disadvantage of Dropbox (much as I love it for other things) is that it's not so good for simultaneous editing; no easy way to merge commits. You also don't get to keep a complete revision history with comments.
I'm less convinced of the value of Google Wave so far; real time typing is almost a distraction, and if I want to discuss math in real time I'm usually best off using the phone, I find.