Think you could skip Rock Band 3 and just cherry-pick the tracks you want to play on your old rig? Think again!
Due to new authoring technology, Harmonix said that all Rock Band 3 DLC tracks will not be playable with Rock Band 1 or 2 software. Rock Band 3 (RB3) tracks include harmony support for vocals, as well as the keytar accessory, in addition to never-before-used lighting and animation cues for the onscreen performance, and all of that extra stuff is unintelligible to the old systems. Also, all tracks that you currently own for previous RB tracks (which Harmonix calls "Legacy Tracks") must be repurchased to play on RB3.
For example, say you want to get a track off the new Bon Jovi DLC pack for RB3 (which John Funk is excited about BTW], but you already own a "legacy" version Wanted Dead or Alive. If you want to grab the Rock Band 3 version of Wanted, you'll need to fork over 160 Microsoft points or $1.99. And if you want the Pro Guitar or Bass format, which will let you dissect the track and play it like a real musician, that will cost you an extra 80 MSP or $.99.
I understand that RB3 offers a lot of new features and uses technology that was unavailable in Harmonix's previous games, but Jumpin' Jimi Hendrix, I never thought that there would be format wars within a videogame. I mean, I hated when I had to re-purchase all of my music when CDs replaced tapes, but you're telling me that I have to do the same thing in Rock Band? That just sucks.
Update: To be clear, you will still be able to play DLC you've purchased with RB1 & 2 with RB3. The tracks just won't have any of the features of RB3 like Pro or keyboard support. Thanks, Dorkmaster Flek and Virgil for clarifying that.