I am developing an application for the Mac as a small team (me + another person) effort. We are located in different cities, and have started to see the need for solid sourc
You can't really go wrong with using Subversion.
If, like me, you don't like Xcode's SVN integration too much you can always choose to use the command-line tools, or one of the several GUI apps like Versions, CornerStone or SvnX. Most of these tools work together pretty well, so you're not necessarily tied in to the tool you start out with.
I personally do most of my work with Versions, and use the command-line tools with the same working copies every once in a while.
If you're comfortable working with command-line tools exclusively until someone creates a good GUI app around it, git is a pretty viable option too.
disclosure: I'm one of the people who work on Versions, so I might be slightly biased ;)