It would be trivial and cheap to make a small dumb bluetooth-serial sender with a reed switch magnet system from an inexpensive wired cycle computer. Then you need software on the N900 side to count the wheel and crank pulses per minute and graph or manipulate it as desired. You can see on ebay that the BT-rs232 can be had under $5 delivered, a processor to handle wheel and crank speed would not need need anything approaching Arduino level hardware.

With the power demands of BT 2.0 it would have to be rechargeable or generate its own power by a magnet/magneto generator, these are moving wheels but not difficult to make and make a 3D printed watertight case. Actually you could have a magnet and magneto loop that both counts turns and uses that electricity to power the unit.