I was browsing the Xbox Live Marketplace, and noticed the track "Less Talk More Rokk" by Freezepop was on the recently added list for Add-Ons.
I've seen this before (Snoop Dogg's Tha Shiznit not too long after the sale - some of his tracks are still two dollars, by the way).
But really, it's got me wondering why some of these tracks are showing up as new. Are they different versions that have small fixes? Perhaps you Rock Band folks with knowledge can shed some light on this quandary.