I have. A while back I wrote to Graham Barr about it and asked if he'd be interested in some code to add a diff feature to search.cpan.org. Today I hacked out a rough demo version. Check it out:
http://sam.tregar.com/modulediff/example.cgi
Suggestions for improvements would be appreciated. You can find the code here if you want to see the gritty bits. Be aware though, this is not finished code.
-sam