Nick - The 5W LEDs are expensive, but cheaper per lumen. That's one reason I am looking at using a mixing chamber vs the approach Huw took. Even higher output LEDs are on the way - one goal of companies like Luxeon is to replace car headlights with a single LED. For my final version, I have a PWM ckt designed (a bit too much like real work - designing analog power converter ICs pays for my photographic efforts ) that will get the voltages and currents where they need to be, as well as dimming.
cao- it would be very interesting to find out what that frequency is. A problem with low cost digital solutions for pulsing circuits is that they are pretty slow. Analog controllers are actually superior in that aspect, as if most of us on APUG don't realize that already