Operating electronic equipment while driving a car is dangerous. Doing it while driving a motorcycle is suicidal. But that doesn't make a GPS any less useful to a biker -- it just makes it less likely you'll get back home without passing through the ER.
The military has attacked the problem head on using helmet mounted Head Up Displays like the one pictured here. They have the money for this type of thing. Several years ago I played with a preproduction unit which was tiny, had an amazing display and an astronomical price -- $10,000US. It could display a full computer screen of data without getting in the way of normal vision. BMW announced a helmet mounted HUD for Ralph Schumacher's F1 efforts in 2002 which likely carried a very F1 development cost and hasn't been heard of since.
Sportview has a $350US limited function helmet mounted display (pdf) for speed, rpm, gear and radar detector alerts which is a move in the right direction but is a long way from duplicating what is displayed on a handheld GPS.
Engadget reports that eMagin is introducing a $600US EyeBud for the video iPod. Tiny, cheap and full motion video. This is the technology and price that could put a GPS HUD on every long distance biker's helmet. eMagin is involved in both military and consumer markets with products that "create a virtual image that appears comparable to that of a computer monitor or a large-screen television." I hope there is a helmet mounted product on their radar screen.
The Seattle PI has more information.