The error message was:

Unprocessed view path found: nil.  Set your view paths with #append_view_path, #prepend_view_path, or #view_paths=.

I eventually traced this down to GLoc, specifically Rails' ActionView::Base#Initialize method that GLoc monkey-patches.
The signature for this method has changed in Rails 2.1; the attached patch fixes GLoc's monkey-patch to use the correct
signature, which resolves the error.

I hope this helps - 
Thanks,
Nick