I have finished my first box of Oriental's warmtone fiber-based paper, struggling with this flatness issue most of the time. I finally decided to shell out the $100 or so bucks for a print dryer - you know, those electrostatic ones B&H sells - and am pleased with the results. While nowhere near as flat as a RC print, they are acceptable, and the dryer accomodates up to 11x14 paper, which is currently the largest size I print. I will be ordering more fiber-based paper now.
Incidentally, a question for the printing experts out there - in Ansel Adams' "The Print" he mentions that the main longevity concern for RC papers is cracking of the resin layer. He mentions that furture improvements may make that a non-issue. Did those "future improvements" ever come to pass? If so, are they real or just marketing hype?