At http://code.google.com/support/bin/answer.py?answer=72765&topic=12044 there is an interesting article describing how you can invoke a Yahoo! Pipe (as the Server) from a GME application (as the Client).

I've turned that idea on its head, with an example of using a Yahoo! Pipe (as the Client) to read a GME application feed (as the Server).

My example Google Mashup at http://lotontechbookcomments.googlemashups.com/ allows the books that I have written to be rated and commented on. Those details are recorded in the ${app}/comments feed. Then my Yahoo! Pipe at http://pipes.yahoo.com/lotontech/cs_book_comments reads the $ {app}/comments feed via its URL
http://lotontechbookcomments.googlemashups.com/feeds/app/comments