Best Mattress for Your Back?

Our same friend that bought CB2's city block bed (we blogged about it yesterday) also purchased a memory foam mattress from IKEA, after doing a ton of research and in-person testing. We hope to soon embark on a similar quest for improving our sleep, and so we'd like imput from all of you on the best mattress for longevity, price, and of course, our back! we keep what we've got an add a memory foam mattress pad on top? We always go back to this great thread from AT:NY about foam mattress pads (which Maxwell then followed up with a top ten list). Anything to add? Best recommedations?

Image of "The Sundance" via Rocky Mountain Mattress