Snopes doesn't have anything on this either way at the time.
If it was true, wouldn't this be all over the news? Wouldn't Fox news be having fits and seizures over it?
It's a bit hard to believe that we'd find out about this from an email a week after the fact.
Edit:
For what it's worth, this guy confirms, and he has some pretty good quotes. According to him, it was mostly a logistical / security issue; Joe Biden did show up.
http://isaacs.newsvine.com/_news/200...ts-not-exactly
Should also note that the prez did attend the Commander-in-Chief ball, which was military-centric.
See what you can find out when you research for ten minutes?
Also, I agree with MothBalls