No, there isn't anything wrong really with the OZ gateway at T-Mobile. The problem is that T-Mobile's spec for the client sucked, and it was built to order!
The server is a good product, T-Mobile just fscked it up by making the bearer SMS for everything, instead of as a failover mode if GPRS wasn't available.
Shame on them.
The SKII has a native AIM (and Yahoo!) client on the device that makes an honest-to-God IP connection to oscar.