This has probably been asked in one of the hundreds "GitHub is down" posts in the last few months, but what are the alternatives (apart from bitbucket)?
I'm a fan of the rarely-mentioned http://repositoryhosting.com/ for the hosting. I don't know a good alternative for everything else GitHub does on top of git.
I used GitLab with a small team for a while and found it to be adequate. It never quite felt as good as GitHub and was missing a few trivial features (although I can't remember any right now) but overall it was a pretty good experience.
Props to you for actually testing simultaneous pushes where conflicts arise at the Dropbox-level, but I would still never do this (for a multiuser repo, for a single user it's probably fine).
Maybe I'm overly risk-averse, but I've been bitten a few times by Git setups that are non-standard or complicated and decided that it's far better to keep things as simple as possible. I'm not really interested in fiddling about keeping Git running smoothly, when it's incidental to the real work. It's just not worth it to save a few bucks, versus hosting a Git repo yourself on a VPS or something.
I run a gitolite server and git-on-dropbox setup both multi user. I find the maintenance of ssh keys a pain. I collaberate with 4 universities outside my country, and people upgrade their computers and lose their keys all the time. If its a smallish, ad hoc, non-security concious project, I go Dropbox by default now.
Its not bitten me yet (1 year, maybe 10 projects)