Finally NBA 2K10 demo is out on XBOX Live worldwide (gold members have exclusivity rights per MS at first, so make sure if you want it right then that you have XBOX Live gold).
If you don´t find it in Demos try going to All Games in the main Xbox Marketplace section and look under "N" ( working in LA ) Probably you find that in xboxlive the Demo is not on new arrivals but on top rated.
Release on the other platforms should follow thereafter, and more news on that forthcoming.
There was a hope that PS3 would get the demo today, but it proved to be false. Uncharted 2 closed beta was supposed to be available today to the public, so we hoped to see a special tuesday PS Store update, but after all, only Uncharted 2 was released and no other demo or content...
Also, official words admit that they still don't know when demo is going to come for the rest of the platforms.