In XCode open Info.plist and look for UISupportedInterfaceOrientations
Add all 4 of the following items and the splash-screen will match the current device-orientation:
UIInterfaceOrientationLandscapeLeft
UIInterfaceOrientationLandscapeRight
UIInterfaceOrientationPortrait
UIInterfaceOrientationPortraitUpsideDown
Tested with Unity 3.4.2 and iOS SDK 4.2