Anyway, I think these days the carbon fork makers have dialed in what it takes to make a fork that's light and responsive yet stable. Try a Profile BRC Road Form ($199) with either an alloy or carbon fiber steerer. Light, but should be fine for fast downhills. So should the Kestrel EMS pro fork, a great buy from Nashbar (www.bikenashbar.com) right now at $179. Either way, here's a trick to try if you don't already know it: On fast descents, clamp the top tube between your knees. That will really help settle down the frame and leads to a more stable high-speed run.
The other question is whether the weight savings are worth it. Steel has some heft to it, but is such a nice-riding material that it almost negates the weight penalty. Maybe the thing to do is set aside the almost $200 a carbon fork would cost and use it as a starting point for a new-bike fund, aiming for something that's aluminum, so you shave weight across the entire frame rather than just the fork. One possibility: Trek's 1200, which has a super-lightweight frame that weighs a mere 2.85 pounds in 56cm, yet costs just over $1,000 for a built-up bike.