Last I heard Cingular did not support the BB messenger.. But it does work on the 8700C and people use it.
Cingular seems to be VERY slow at putting anything out.. OS updates, the BIS 2.0, Yahoo IM and NOBODY there seems to know anything when you call or write the forums: http://forums.cingular.com
Its sad that you pay more ($20 more) and get less then others. I wish I was not stuck in a 2 year plan.