An interesting idea was proposed over on the OCForums involving counter rotating fans improving static pressure significantly with minimal additional noise. Posted along with this was an article by a Sanyo Denki employee from the Cooling Systems Division that explained the mechanics of it.
http://sanyodb.colle.co.jp/pdf/16e/b.pdf (Skip to page 3)
Pretty interesting stuff. This got me thinking that it might give better results to use two counter rotating 120mm fans instead of sandwiching the heatercore or radiator with a push/pull setup. Of course, there remains the difficulty of finding counter rotating fans, unless someone knows how to disassemble a fan and reverse the blade direction.