iOS doesn't actually spawn a new thread for every window in an application. All of the UIKit calls are still very thread-unsafe and expect to be used only from the main thread. This doesn't really cause much of a problem, since apps are page-based: you don't need to update more than one or two windows at one time, anyway.
Yeah, for anyone who doesn't believe this, do a search for _WebThreadLockFromAnyThread sometime. All those Stack Overflow questions are from people who thought it was safe to have threads anywhere near UIKit.
