An arduino with 8 LEDs is easy. The hard part of doing a project like SpokePOV or Hokey Spokes is supporting more LEDs and synchronizing multiple spokes.
Synchronizing multiple spokes is needed to obtain persistance without spinning so fast. Supporting 25-30 LEDs is difficult because of the wiring. How do you wire an atMega, 4 8-bit latches and ~30 leds without a PCB? The wiring gets a bit messy.
And you can't ignore cost. With three spokes per wheel, you are looking at 6 spokes per bike. I've got 4 bikes.
Some kind of open source pcb, group buy is needed...