The Rock Band 2 registration site is now up and running for those looking to get their 20 songs via download.  While the songs aren't available yet, you can register now, so Harmonix can e-mail you the download code when they become available in a few weeks.  Also while filling out the form, it has a comment box where you can suggest Bands & Songs you would like to see on Rock Band 2.  Unfortunately, no word on what will be included in this 20 song DLC as of yet.  I personally hope they throw in some J-Rock, but I know it's all wishful thinkin.

Also, let us know what bands or songs you hope are in the 20 song DLC, or just some you would love to see featured as DLC later on!