I might do that at home if I had a bike. (but not at the gym though!) I can't imagine getting that much of a workout in, but I spin so maybe a regular bike isn't as intense. However I wouldn't consider it a real workout, just something to do on my off days perhaps.
I've seen people reading on a bike which isn't that odd (neither is knitting really)
The stuff people do at the gym is crazy sometimes. Like the one girl at the gym that turns off all the fans before class starts. What's up with that?