Yes, since we have unlimited data, it will not be a good thing if we have to send IM using SMS.
On the other hand, I know for IM+, you can actually use yahoo to send SMS to mobile user (that don't have IM), and that way, you don't have to pay for SMS cost, and also can still chat with other that don't have messenger service on their phone. THis will be a good feature.
ANother thing I would like to see is the abilities to do conferences (i.e. multi people chat).
Yes, JiveTalk is good, but anything free is even better