Google have been working on a tooling library for Clang. In since the 3.2 release. It includes a ASTMatchers library so you can just build up a query and don't have to walk the AST.
There is a great video talk on the subject that walks through a simple rename example. (This is from the same guy as the MapReduce talk posted above but is newer and more about a simple practical implementation rather than the internal design and enterprise scale stuff Google have going on).
The source for that example that renames a method is available in the tooling branch. It may be somewhere in the trunk but I can't find it. Also Rename the getDeclAs function to getNodesAs as the other is apparently deprecated.). There is a more advanced example that removes duplicated c_str calls (which is in trunk and someone posted above).
Here is documentation for LibASTMatchers and LibTooling.
EDIT: Some better docs for ASTMatcher. Here and here.
EDIT: Google are now working on something called Clangd which aims to be some kind of Clang server for refactoring.
