There is a planned 12 pack coming this December with more on the way in spring 2011.
http://www.nintendoworldreport.com/news/24339
Harmonix confirmed a report that Billy Joel is coming to Rock Band 3 on December 14.
The 12 pack of songs, dubbed the "Hits" pack, will feature songs such as "Piano Man," "Movin' Out," and "Big Shot." It will be followed by a "Piano Challenge" pack, which will include six more songs from Joel.
See below for the full list of confirmed songs.