I surfed over to the Presidential Inaugural Committee website and discovered news there of a "Neighborhood Inaugural Ball." There's not a lot of information about it yet, but here's a rundown on the most important points:
The Washington Convention Center is ugly (see photo above) but MAMMOTH. It is the site of 5 different Inaugural balls that night, according to the New York Times. It's 2.3 million square feet, with a 52,000 square foot ballroom.
I called up the Convention Center's spokesperson who says that the ballroom space for the Neighborhood Ball can accommodate 5,000 people, though she was unsure of how many the Presidential Inaugural Committee actually plans to bring in.
By the by, through January 8, there's an open contest to win tickets to the Inauguration. You sign up here and have to write a little essay about what the Inauguration means to you. Good luck!
Obama to Attend ‘Neighborhood Ball’ [New York Times]