In native way, apple suggest something like this

- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration
{
    if (UIInterfaceOrientationIsLandscape(toInterfaceOrientation))
        self.bannerView.currentContentSizeIdentifier =
            ADBannerContentSizeIdentifier480x32;
    else
        self.bannerView.currentContentSizeIdentifier =
            ADBannerContentSizeIdentifier320x50;
}

I have checked the document (http://developer.appcelerator.com/apidoc/mobile/latest/Titanium.UI.iOS.AdView-object.html)
but no luck to find some properties like currentContentSizeIdentifier.

P.S. There are properteis SIZE_320x50 and SIZE_480x32 so I believe there are way or titanium planing to do it.

Any suggest is welcome. Thx