[stuntmouse]

You can write native Cocoa apps with MacRuby that have Mac Look and Feel, with better than acceptable performance.

There are clean, high-level Ruby wrappings of UIKit classes, see HotCocoa: http://macruby.com/hotcocoa.html, although it's not clear to me how extensive this part of MacRuby is.

[Zev]

Just to nitpick, its AppKit, not UIKit. UIKit is on the iPhone, where, sadly, MacRuby isn't available.