You'd have to make sure your app is Universal (of course) and this is how I load my credits modal dialog...
Code:
BOOL deviceIsPad = (UI_USER_INTERFACE_IDIOM() != UIUserInterfaceIdiomPhone);
AppCredits *appCredits = nil;
if (deviceIsPad) {
appCredits = [[AppCredits alloc] initWithNibName:@"AppCredits_iPad" bundle:nil];
[appCredits setModalPresentationStyle:UIModalPresentationFormSheet];
} else {
appCredits = [[AppCredits alloc] initWithNibName:@"AppCredits_iPhone" bundle:nil];
}
[self presentModalViewController:appCredits animated:YES];
This shows the modal view full screen on the iPhone and in a smaller Presentation Form Sheet on the iPad.
Hope that helps!