A couple of people suggested that clipping the Fitbit to your shoe would help. So over the last week I conducted an experiment. My ride from home to the office is 6.5 miles. With the Fitbit clipped into my hip pocket it registers 2,362 steps. Perched precariously on my shoe I get 2,389 steps. You can't cheat Fitbit this way.
How far off is 2,300 steps? If I was walking the same distance I'd get 13,000 steps. But I wouldn't be coasting down any hills so that isn't right either.
For my weight relaxed walking should be around 155 calories a mile, cycling at around 10 miles per hour is 78 calories a mile. This is about a 2:1 exchange rate so those 2,300 steps should be 4,600 or so.
Fitbit does have some options to manually add activities that it doesn't register correctly. This sounds too much like hard work though, it's difficult enough to remember not to put the poor device in the washing machine. I'm also 234,037 steps behind the current leader for January so I'd need to cycle home for lunch as well to stand any chance of catching up...