So T-Mobile released OS 4.0 for the BB7730 and BB7230 about a week ago. I installed it on my BB right away and it works fine for me, although unlocking it is a little slower than before.
Has a lot of great features that are new, such as over the air synching of all data, much greater font choices, password keeper availability, over the air enterprise activation and so on.
Problem is, it also takes up about .5 to 1mb more of memory on the BB. So if you have someone with a 7730 that is already near to capacity with OS 3.7, I would suggest that you be very careful when upgrading to OS 4.0. Do not do a straight export/import of their data, but rather move only the data that the user needs. Alternatively, you can wait until the 7290s are available sometime within the next month, as that unit supposedly has 32mb of memory instead of 16mb for the 7730.
7290s and the similar 7250s are already available from some other carriers.
I'm hoping that RIM will do an update soon for the 7730 to include quad-band, bluetooth, faster CPU and more memory.
By the way, for those looking for differences between Blackberries and Treos, apparently the Treo processors are something on the order of 10 times faster. This can make a big difference for those of you out there with large contact lists (more than 1000).