The kind of things that show that you went the extra mile to add polish to your app.
I like that article. But I actually get designers to construct UITableViewCell in Interface Builder. It works out better than creating cells programatically and leaves the design work to actual designers.
I've found that keeping an eye on iPhone UI design can help out with UI design in other areas if you find yourself in a space-constrained situation wanting more out of your controls or existing UI.
Smashing Magazine does this thing that really irks me, where they pick a bunch of good examples and then one or two very mediocre ones. It leaves me wondering if I'm insane or if Smashing really is making an effort there.
The Convertbot UI makes me cry. But apparently the eye candy sells.