<p>I recently downloaded Mono 2 for my new MacBook.  I'm trying to figure out how to compile Visual Studio solutions with xbuild, which is included in Mono.  Any ideas?</p>

The last time I checked, solutions weren't supported by xbuild (they use a different file format), but *.*proj files are.
If you have a relatively simple solution, you may be able to get away with using xbuild on each project separately, but if your solution is very complex (with interdependencies between projects, etc.), you'll probably have to do a bit of hacking to make everything work.
Furthermore, it's worth noting the C++ projects in Visual Studio don't use msbuild, either (although Visual Studio 2010 is supposed to fix this), so if you need to compile any C++, you're totally on your own (of course, Mono doesn't have a C++/CLI compiler, so you wouldn't get very far with xbuild, anyway).
This question/answer
suggests that you may wish to use mdtool, instead.