This story against iOS's close buttons is getting more attention, and while Gruber's argument against it (that gesture-based solutions are non-obvious and therefore insufficient for iOS) is sound, I think a more important criticism is that the entire point of the close buttons in iOS is that they are difficult to use, intentionally.

For the tab closing, app-quitting, notification-center-clearing activities in which the close button is used, there is no going back - these operations don`t respond to "shake to undo", and there's no command-z available. These also represent instances where there may be significant data loss for a mistaken trigger. If I close an active Safari tab with unsaved work in a form, that's gone for good. If I quit an app that's still processing something for me, oh well. And if I clear my notifications in error, it's back to the bad old (recent) days of news lost to the ether.

That's why I think these buttons exist and operate the way they do - in accordance with what Jeff Atwood called the Opposite of Fitts' Law:

Don't forget to follow the opposite of Fitts' law, too -- uncommon or dangerous UI items should be difficult to click on!

Closing these items should be more difficult than opening or creating them, and should require deliberate effort.