I think it would be extremely ill-advised to pull out a knife when you're outnumbered. They could all have knifes, or worse, guns, and suddenly you give them reason to use them instead of simply taking your wallet and giving you a bump on the head. I think your best course of action would be to give them your wallet, no, THROW your wallet as far as you can away from you, and play dead.

While I didn't have a knife pulled on me personally, riding the T home yesterday the guy sitting next to me pulled a knife on a someone who he thought was slowing down traffic. To be fair, he was slowing down traffic and we're very uptight about traffic here in Boston. That was approximately when I called 911 on my cell. The police never showed, but nobody got stabbed, either. *shrug*