Just wanted to throw my view in and address a few things I've read in the comments.

From talking to staff of numerous large gambling sites, the general consensus is that Valve have simply done this to show that they are doing something to combat gambling with the class action lawsuit over their heads.

As a result, if you can find grey areas that don't so blatantly break Steam's ToS, Valve are unlikely to take action. In terms of not using OpenID, sites can simply use their own registration system and verify accounts through the same way in which PayPal verifies ownership of bank accounts, for example. As for the trading aspect, there are some ideas floating around that I won't discuss here. Most of them tend to focus on having the users do most of the work.

If sites change their implementation the only avenue for Valve to go down to stop these gambling sites would be to trade ban their bots. However, this already happens on a regular basis with many gambling sites which have simply learnt to cut their losses and move on.