Here's a nice little article about the effectiveness of long cranks for tall riders...
Goes some way to proving what I've been finding on my bike.
I've had the go ahead from a university here in New Zealand to do a power output test of long legged riders with varying length cranks. I have to find 8 riders over 6'5" now - which is not exactly easy... Be interesting if I can find them though. And make for a good photo if we can get all of us out for a ride