The revisioning system being designed and planned for 1.0 release will include the source of all "content elements" managed in the database. That includes snippets, chunks, plugins, modules, TV’s and the main content.
Also, just as an aside, I currently avoid this by always keeping my snippets in external files. In other words, the only source code in my snippets is something like:
return include ($modx->config['base_path'] . 'assets/snippets/mysnippet/mysnippet.snippet.php');
I then keep the snippet files in SVN...
This has the added benefit of significantly reducing the size of my siteCache file, since it reduces the amount of source code stored inside of it.