I would like to be able to thin the UITabBar's height by removing each item's titles and reclaiming the vertical space they take up, a la Tweetie 2. This doesn't seem settable in the .xib or programmatically. Will I have to subclass the UITabBar and roll my own?
Roll your own. I'm sure Tweetie 2's is custom; and the height is set in the handed down UITabBar.
You can achieve a similar effect by placing a segmented control inside a uitoolbar. I did this and it looks quite nice.