I'm kind of keen on doing something similar to this, and have thought about it extensively. I'm really only going to be building a speedo though.. but I've had a couple of thoughts you might consider:
It's starting to seem like a popular project 
- Touchsense/Capsense is MUCH harder to implement if you are wearing gloves. I've had a crack at this and it's not worth considering I reckon.
It depends on what your sensing range is like. You don't want the button too close to your hands with a larger sensing range or it'll be sensing your hand all the time but it should work through gloves no prob. Unless the gloves are shielded or something
- Accelerometers are also quite hard to implement with lean-angles as there is v-little lateral force due to cornering if the accelerometer is fixed to the bike. Depending on where the acc is mounted along the 'north-south' axis of the bike, all the forces due to cornering will be directed down through the Y-axis. You COULD calculate it using vector geometry, and I managed to using a Nunchuck, but it's still quite tricky.
I am no runner from a difficult bit of math but I don't think it would be of any use, it's just another random thing that could be added...
- Gear indicator is easier to implement if you are reading both speed AND tachometer values. Just have some sort of calculation... if you are doing 100km/h and the RPM is 2000RPM, the you're in 5th. If it's reading 3400RPM then you're in 4th etc etc.
This thread is about adding an arduino to a pedal bike. It is much more difficult on a pedal bike due to the front chainset. If it was single chainset on the front then it would be easy to measure. It is also rather pointless on a pedal bike because you usually know what gear you're in unless you have a rubbish gripshift of a broken gear changer...
- Thought of using something like a PS2 controller, or Wii Nunchuck (which is i2c!) which has the analog joysticks on them/it? Waterproofing it would be tricky, but how cool would it be to have a PlayStation controller on your bike?
Yeah I haven't seen any waterproof nunchucks either... Interesting idea though because that would provide a joystick and accelerometers with an easy to use interface...
I was contemplating using a flexible (and waterproof) PS2 numerical keypad and assigning each 'display' a different number on the pad. I was also thinking of adding a PS2 keyboard port so if I get round to implementing this on a trike, I could type messages when stopped and have them scrolling on a matrix display on the back of the bike.
Looking forward to hearing more about your trials. I'm doing this from New Zealand, so if I'm buying any nice stuff from Sparkfun I have to pay lots for freight (Dammit)
I'm in the UK so it's not too cheap for me either...
Mowcius