Sonic Adventure 2 and Nights into Dreams will hit XBLA and PSN in the first week of October. I'm assuming that Steam will get them around the same time.

http://www.neogaf.com/forum/showthread.php?t=491761

It'll be $10 for both. The Sonic Adventure 2 Battle add-on will be $3 extra, and Nights will include the Christmas levels free.