After getting yanked off of Apple's App Store a little while back, Blizzard's Mobile Authenticator app is now back in business, and ready for a free download. Version 1.0.2 is out now [iTunes link] and the description says it contains UI improvements with a streamlined and enhanced interface.
Unfortunately, the issue that originally got the app pulled off the store hasn't actually been fixed: the official blurb is now saying that you should definitely remove the authenticator from your account before you upgrade, and then re-apply it again to your account after you've upgraded (and presumably gotten a new key installed). If you install this new version of the app and then try to access your account, it won't work (and you'll have to call Blizzard support at 1-949-955-1382 to help them remove the old authenticator).
Small hassle to go through, however, to have an account protected against hacking. If you have an iPhone or an iPod touch and haven't picked up this application yet, now's the time to do so for sure.
[Thanks to everyone who sent this in, especially Eric!]