The Rock Band Network Store is to be ported over to the PS3 version of Rock Band 2, according to this Joystiq report, although Wii owners won’t get a look in at all.
The facility allows users to buy tracks made in the Rock Band Network interface, which is made in XNA and therefore can only be used on 360.
There’s a bunch more through there. Also, there’s impressions of Rock Band Network’s tools here. Warning: not for the fainthearted.