speaking from experience, the time and effort you will put into trying to use a standard digi cam and fabricating mounts is NOT worth it, considering goPro hero HD are now sold at Target, in stock, on the shelf for 295 bucks.
These days for the cost of 1 goPro, 1 copy of Sony Vegas movie studio HD, and a little creativity, you can be a regular Martin Scorcese. But trying to mount a digi cam to your bike with some home depot shit, not worth it, especially when you see the shake/howl of the finished product.