My old friend Tony has it right when he says:

Some commentators have suggested he straightforwardly implied that Palin was a pig. Which would be insulting, but doesn't sound sexist to me. A more subtle (although more sensible) jab would be the following implied argument: 1. A pig with lipstick is still a pig 2. Palin (according to her acceptance speech) is a pitbull with lipstick 3. Palin is still a pitbull That's arguably unfavorable, except that Palin made the comparison herself.