What's the '(' for? It should be something like:
Are you sure you really want to /dev/null mail from any address at yahoo.com?
I think this is probably unnecessary.
Is this actually:
or do you really just have ':' there?
If you're still having problems, check your permissions on all procmail related files (make sure they're not group or other writable), and turn on logging so that you can see what's happening. Turn on verbose logging if necessary (VERBOSE=yes).