I keep on meaning to have a look at this, but I don't currently have a dynamo on a bike, so it's not too big an issue.
If I was going to build one, I'd probably use some sort of switching regulator to bring it down to 5V, with a crowbar on the output to protect the device if anything went wrong. I need to pin down our power supply bod, and pick his brains!
One issue could be that many of the devices which you would want to power, have their USB connectors hidden behind rubber, waterproof covers. Having them plugged into a power connector, whilst in use, will negate the waterproofing quite seriously.