1个viewController怎么关联2个XIB，一个是iphone的XIB，一个是ipad的xib 

if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone) {
YourViewController *vc = [[[YourViewController alloc] initWithNibName:@"YourViewController_iPhone" bundle:nil] autorelease];
}
else
{
YourViewController *vc = [[[YourViewController alloc] initWithNibName:@"YourViewController_iPad" bundle:nil] autorelease];
}


Nib names that include a platform-specific identifier such as ~iphone or ~ipad are loaded only on a device of the corresponding type. For example, a nib name of MyViewController~ipad.nib is loaded only on iPad. If your app supports both platform types, you must provide versions of your nib files for each platform.


详见： UIViewController/nibName

即准备2个XIB文件，文件名中使用不同的后缀(~iphone/~ipad)：

YourViewControlle~iphone.xib
YourViewControlle~ipad.xib

应用代码用统一的一行：

YourViewController *vc = [[[YourViewController alloc] initWithNibName:nil bundle:nil] autorelease];

因为是参数都是空，在有ARC的Objective-C下，上面的代码可以写成：

YourViewController *vc = [YourViewController new];

试试看～