LEGO. RockBand. Two great licenses that taste great together. The music selection is pointed at children and families, but thanks to the use of some older songs in recent children's movies (such as Kung Fu Fighting in Kung Fu Panda), some of the songs are likely to be familiar to parents, as well.
One question that is on everyone's lips, based on my conversations with the booth's handlers at E3, is whether the DLC from other RockBand games will be playable in LEGO RockBand and vice-versa. I asked the question, myself, but was told that the answer to this question is not yet being revealed.
While it would makes sense to make the existing library available, I wouldn't be the least bit surprised if it's not, for a couple of reasons. First, there are licensing issues involved here. When musicians or labels agree to let their songs be used in a game, they're not signing away the right to use the song in as many games as you please. And, there is a legal agreement than involves the music license holder getting some sort of payment in return for the use of the song. So, just because the song is made available for one game doesn't mean it will necessarily be made available for a subsequent game. A second reason the existing DLC library might not be available is to constrain the music selection in LEGO RockBand. As I mentioned above, this game is being aimed at the family crowd and children. Not intermingling the DLC is one way to make sure that the songs that can be played in LEGO RockBand are appropriate for LEGO RockBand. For that matter, the folks at LEGO may object to certain songs in the existing DLC library being playable in LEGO RockBand.
The point is, it could go either way, but there are a lot of "legal" issues that make me lean towards not expecting to see full DLC library availability with LEGO RockBand.