Argh. Had I known beforehand that the free version of Unity3d purposefully created its project metadata in a way that makes it incredibly difficult to version control, I might have ran away from it. Well, actually, it's not the version control that is hard (Git will happily track binary data) but the sharing and merging with other changes.
This all means that I will have to be extremely careful should others want to help with the project- to the point that it might just be easier to have people email me the changes so I can directly add it.
Unity Pro, which can make all the project metadata text based, costs $1.5k. Waaay out of my budget.
Oh well, I suppose I'm putting the cart before the horse a bit here... I don't yet have any people offering to help anyway, so this hasn't really affected me yet. But it's annoying nonetheless.