Here are a few ideas for carrying a GPS when you ride...

Outdoor Products makes the perfect case to hold a GPS, cell phone, FRS radio or any other accessory. The padded compartment attaches securely to any backpack shoulder strap with 1.5" velcro. Non-slip grip keeps case in place. This has worked great for my eTrex Summit and especially the Rino. Size: 7.5in. H x 3in. W x 2in. D.
I've also used the Garmin eTrex series Neoprene case for my Summit. It has a belt clip on the back that can be clipped on most straps, but be sure to tie the lanyard in case it vibrates free. I've found mine dangling from my pack after a bumpy day on the trails...which made me look into another alternative (the accessory case by Outdoor Products).
I haven't gone to any of the dashboard or handlebar mounts for my GPS only because I wonder about vibration. Several people with mounts have told me their units often turn off for one reason or the other related to getting jolted around on rough trails. With the device on my body, I've been able to avoid any of these kids of failures. Although, I'd still like to wire it to the machine to solve battery issues.