How to show git files in Finder?
An update, as there's been movement on this question...

There is a question on SO: How can i add icon overlay in finder? where a comments include Apples update of  points to the TortoiseSVN Subversion client 
and a search turned up the following 2yo git repo with some demo code for 10.7: Finder-Icon-Overlay.

If I get around to messing around with an implementation, I'll update this answer.