If you signed your name on an actual contract, you're liable for the contract. If, on the other hand, you're an idiot and sign it with your Second Life avatar (or Slashdot ID for that matter), I would imagine the contract is at least called into heavy question.
I did contract work in SL for 3 years. I always signed my name on real, mailed-over contracts. I had to do work with other contractors, though, who in a fit of privacy histrionics, refused to divulge any part of their private life to these real-world companies they were working for, and thus "signed" a "contract" inside the virtual world.
Not surprisingly, they either didn't last long doing contract work or got so heavily ostracized for their insanity they never got another call again.
In short, don't be a moron. Get a real contract, in real paper, and sign it with your real name (and make sure they do too!)
Anything else is just roleplay.