Like many of the other services, Crystal is offering to integrate challenges, achievements, leaderboards, and other social services into iPhone apps. The SDK seems like it's still pre-release, however -- there's only a signup, no information on cost or implementation or anything else on the site. Still, if you happen to be an iPhone developer still looking for a network to hook up with, they might be the one for you.
While we're at it: what do you consumers think? Have you actually chosen a network to go with, or are you doing what I'm doing and still basically going game by game? The goal of these networks eventually is to have a unified stable of developers, where you'll jump into a new game simply because it's linked to the network you're signed up with (and your friends will be playing over there and encouraging you to join). But in reality, I haven't seen that -- most people I know are just playing the games they're interested in, and the network the games are connected to hasn't made a big difference.