The question would be how does someone actually prove they are bike commuting. I remember reading some statistics last year indicating that ownership of bicycles has never been higher, yet the number of miles actually ridden is as low as it's ever been. Thus mere ownership of a bike wouldn't be a suitable criteria. It's just too open to abuse.

Personally I'd be changing the tax laws to eliminate the tax rebates that car commuters get. Mind you, don't expect it to make any difference anytime soon either way. The bicycle is by far the cheapest and most reliable way to travel now, yet so few people use it. I'm really not sure tax breaks would make any difference.