SourceTools is good once you're up and running, but if you're needing to export from a large number of Excel workbooks, opening up each one and exporting it could be a bit tedious.
VbaDiff (disclaimer: my product) has an API that can read through multiple Excel files and extract the VBA code. There's an example of here - it could easily be adapted to export the code to a file, ready to be checked in. If you're good with SharpSvn, you could probably add the code to the repository as you go!
