Sorry. I'm too lazy to read through these 3 pages to see if anybody's suggested this yet. Arkel's Switchback sounds like it's what you're looking for.

http://www.youtube.com/watch?v=2m9C_7DVAYQ

I use a Tubus Ergo rack and Lone Peak P-150 panniers and they're great, if either of those interest you. As far as the Ergo not being stainless, it is powdercoated, so instead of lasting forever it should last almost forever!

The switchback is not waterproof. 99.9% of the time I'm a fair weather rider, so I have the luxury of staying away from the potentially musty/moldy waterproof bag situation. My cell phone, camera and GPS all have Ziploc bags ready for them if it's ever needed, water problem solved. Happy gear shopping!