Recapturing Bike Energy from Gyms

Why are we not using gyms has power plants?

I first came across this the idea of gym power plants at Whynot.net and then recently here at treehugger.com

The basic idea is so simple...

1. People go to gym
2. People ride stationary bikes (spin class anyone?)
3. Bike is hooked to a generator
4. Gemerator creates power
5. Power is push out into the grid
6. Earth is saved!

What do you think?  I know it wouldn't be gobs of electricity but I think if we can tap into more of these simple unused sources, we'll be better off in the long run.