I have a 430 EX mounted on a 450D/XSI.
Is there any easy way to quickly disable the flash for a shot, without turning it off, as you have to wait for it power up again.
430EX does not instantly discharge when you turn it off. Therefore if you do it just for a shot and then turn it back on, it'll still have almost full power and be ready in almost no time.
The 450D does indeed have a "no flash mode" on the dial but as John states it's one of the auto modes so you can't select aperture etc.
There is no way to disable the flash through the camera menu unfortunately on the 450D, the xxD models and above do have a custom function to stop the flash firing (handy if you just want the AF assist lamp) but that's not going to be much faster than turning the flash unit on and off.
I don't have a 450D, but doesn't it have a mode on the dial for flash off? It would be fully automatic, I would guess, so that may not be desirable. The only other option I can see is disabling it in the camera menu, but that may be a little too slow for your purpose.