In case tkanzakic answer didn't work, you can always use a UIView to put the banner into and add it to your view. In banner load delegate, resize your intermediate view to banner's bounds.
edit: Something like
ad = [[[RevMobAds session] bannerView] retain];
ad.delegate = self;
[ad loadAd];
- (void)revmobAdDidReceive {
intermediateView.frame = CGRectMake(0,0, somewidth, someheight);
ad.frame = intermediateView.bounds;
[intermediateView addSubview:ad];
}