I started a thread about the same thing:
http://www.gpwa.net/forum/always-use-rel-nofollow-affiliate-links-172174.html
I found that all of my targeted keyword phrases SERPs were dominated by affiliate programs. They dominate the highest competitive keywords NOT because they write fresh new content daily (I don’t know any that do), but because they have countless incoming link via banners and text links in their affiliate codes. Google counts them (yes even banners) as an incoming link, which then propels them to the top.. even above you!
So with that in mind get a grip on your link flow! Use an internal redirect with a rel=”nofollow” in the url.
You will not be penalized for doing so. (So says matt Cutts http://www.stonetemple.com/blog/?p=187 )
I’d also like t mention to NOT display your casino banners that predominately display their website name. this will lead to direct type-ins. I don’t know what the percentage it, but any is not good.