Over on his blog, Mugunth Kumar has been covering error alerts on iOS. I covered this as part of my LiDG talk on Error Recovery not too long ago. Haven't gotten round to putting the slides up yet, but will soon I hope!
Displaying error alerts is one of the few areas where OS X developers are fortunate enough to have a leg-up over iOS. We can simply call -presentError: or -presentError:modalForWindow:delegate:didPresentSelector:contextInfo:.