A new service lets Twitter users monetize their tweets with AdSense... http://www.moneywithadsense.com/2010/10/mylinksmyads-marries-twitter-adsense.html
It's a violation of AdSense Policy to place ads over someone else's framed content. https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=105956