A 'highly-placed source close to Harmonix' has revealed that the tracks included in the up and coming Lego Rock Band will be exportable, giving you the ability to play them on your Rock Band 2 disc, the same way Rock Band tracks can currently be used.
We have also been given a little bit more detail on upcoming DLC due at some point by the end of the year. Though not all track titles have been confirmed we can see the likes of 12 Queen tracks, 3 tracks by The Raconteurs and 3 from The White Stripes on the way, to name but a few.
Rather than attempting to give you all the information Wired.com has, I would recommend you check out the insanely detailed chart that they have created on the page linked below. This compares the Lego Rock Band, Guitar Hero 5 and up and coming DLC tracks.