Don't fret, it's not to late to find a geeky physics Halloween costume! GeekGirlGifts and Cocktail Party Physics suggest going as Maxwell's Demon or Schrodinger's cat.
If your want a easy costume that doesn't involve a whole lot of effort, throw on all things black and be a black hole.
Or how bout a neutrino? Act neutral and wear a sign that says "I barely have mass".
If you have the time and inventiveness, construct a Mars Phoenix Lander costume - be sure go around telling everyone you've discovered snow.
If you want to make a lasting impression, enter the party spinning rapidly (and stay that way). If anyone asks tell em' you're a pulsar.
For something more contemporary, be the main ring of Large Hadron Collider! Attach a giant hula loop with a bunch of magnets to yourself. You could even figure out a way to circulate "proton beams". If you've had enough partying, just tell your friends a magnetic quench shut you down for the night...