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.