I know many people who use Cingular/AT&T have not been able to get Y!M to work on their BB without doing the crazy workaround. Well, a buddy of mine who works for Cingular/AT&T, called me up to let me know that Y!M is now good to go for all BB devices.
HOWEVER, I am still using the workaround and have not tested to see if it is indeed working - so, good luck!