He just fell off the stage trying to dance like Michael Jackson and got banged up.

Here's the video:

he's just sore...

"He hit his head and back. No concussion, stitches in head and back near shoulder. Broken shoulder. Yes he is very sore. We were all worried," Perry continued. "Once he left, it was diifficult for people to contact one another due to bad cell service it made things even worse. The boys r back in Boston. ... Steven will see his own Doctor tonight or tomorrow."

http://www.mtv.com/news/articles/1617848/20090807/aerosmith.jhtml?rsspartner=rssFeedfetcherGoogle