Google Code Search
At the moment Google appears to be spidering CVS repositories and archives (ZIP files etc.) It surely won't be long before Subversion is included in that list since that is what Google Project Hosting uses!
Lots more info on Google Blogscoped including:
Additional to the regular web interface, Google Code search also comes with a REST API (simple parametrized GET queries returning XMLs). This way you can build code search functionality into existing programs (like IDE’s). Here’s the result XML for the sample query “hello world”.
Here is an example search for files in the NUnit package on Sourceforge.
Update: It looks like it does already support Subversion as you can see from their code submission form.
I like the CVS example. :)