When I was last on the phone with Tmo while trying to get my Curve to work, the girl I talked to was very jealous and said that they would have it for sure before the end of the year, because they were already doing training for it. Common consensus around here is that it will be late Aug or early Sept.
Just a sidenote, RIM and Tmo weren't much help getting my Curve working when I initially got it. I had to turn to Pinstack and they had the workaround all laid out. The guy I talked to at RIM essential said "go check Pinstack" without actually saying it.