Here's what I'm seeing in Xamarin Studio 4.0.4.
Right click on a project and select Options, then go to the Build > IOS Application section and select the Advanced tab. If you already have one or more document types, just going to this screen will throw an exception for each one. Since these exceptions are handled gracefully, you still get to the Advanced tab, but it doesn't show any of the icons for the document types, just empty placeholders that say (58x58). And it won't let you specify an image that isn't 58x58. Unfortunately, according to Apple's documentation (http://developer.apple.com/library/ios/#documentation/general/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html#//apple_ref/doc/uid/TP40009249-SW1), document icons are 22x29 and 44x58 for the iPhone/iPod and 64x64 and 320x320 for the iPad.
Is this a bug or am I misunderstanding something here?