Nick - I have 2 green and 2 royal blue 3W LEDs. The LEDs were within one stop of the 150-250W bulb with #2 Kodak filter, that it replaced. Neither the bulb or LEDs are close to the light intensity I'm planning for my final source.
My plan is to make a big mixing box for a diffused source. Although bulky, the size covered and light intensity can be modified quite easily. That said, scaling Huw's design and using 3W instead of 1W LEDs should get you similar performance for 5x7, and the same for 5W and 8x10.
Bob F.- a 555 timer should work fine to PWM, but without a means to regulate the current, the circuit will just vary the effective voltage. Adding current control gets you much of the way to switching regulator I'm using an LM10 set up as a V-I converter - same level of complexity as a 555 circuit.
A red Lumiled works very well as a safelight and since it doesn't matter if they vary by 1/4 f-stop, a wall wart and resistor work very well to drive it. I've used one for a few years without issues.
OT question: Is there any way to change my name on APUG from 'jking' without rejoining? I'd like to change it to my full name, but haven't seen how to do it. Thanks!