Not to be outdone by yesterday's Characters app for the iPhone, Michael Gaines has posted his own WoW app to the iTunes App Store, this one tracking server status easily and quickly in iPhone format. WarcraftStat is as simple as they come -- you get a list of all the servers, their PvP/PvE designation, their population label, and the little arrows found on the usual realm status page showing whether you're good to go for ganking noobs in Westfall, or the realm is still down.
Unfortunately, all of the realms are a little much -- I'd kind of rather choose a couple of pertinent realms to me and only see those on the page. And notification of when a realm goes up or down might be nice, too, not to mention that the app is currently US realms only. But as an app that tells you whether realms are up or down, WarcraftStat does its job. It's on the App Store right now, and available for a free download.
[ Thanks, Brad! ]