My n800 doesn't have gps. Bluetooth for a bike computer is pretty cool. $80 for a bike computer is pretty high. But an app could log huge amounts of data, produce graphs and charts, and so on. I'd want a really, really, really good mount for my n800 for my bike, so i could see how fast i'm going while riding. The n800's screen isn't very readable in direct sunlight... and i wouldn't want to subject the unit to direct road vibration.

Or. One could use flite (text to speech) on the n800 to tell you while you ride. Then, keep the unit in some pocket, with a headphone. The n800 could be playing other audio, and the bike computer app could mix this in, perhaps just in one ear. It might just tell you your new speed when the speed changes or falls below some threshold. With audio in, you could give it voice commands to ask for specific things.