The success of extending the qualifed transportation fringe benefit to bicycle commuters (sec. 126 of the bill and sec. 132(f) of the Code) will be partially determined by how many people can use it. So how many did Congress think would use it? We haven't found a reference to a specific estimate of the demand but the Joint Committee on Taxation scored the bill (i.e., estimated impact on revenues) at ~$1 million per year for 10 years. http://www.jct.gov/x-78-08.pdf The methodology JCT used isn't public but if one were to assume $1,000,000 of foregone tax revenue from people claiming the $240 maximum and having an average effective tax rate of 15% then about 28,000 commuters might expect to use the benefit (this excludes the payroll taxes that employers would not have to pay on the $240, too).
The number of commuters might be much higher if the average claim was much less. Of course, participation depends on how many companies make the subsidized bicycle commuting benefit available to employees. Bureau of Labor Statistics reports about 5% of workers have access to subsidized commuting. At the same time, the recently release 2007 American Community Survey found about 0.5% of commuters or over 664,000 usually bicycle to work.
The question then turns to are there particular socio-economic groups who will benefit more from this benefit? ACS 2007 shows that 77% of the bicycle commuters are male so one could argue this proposal could benefit more males than females. The Census' American Factfinder combines the number of people bicycle commuting with taxis, motorcycles and other means for most of the socio-demographic comparisons (e.g., race, income, etc.) so we are unable to tease out the socio-economic factors for only bicyclists. Additional work the ACS PUMS dataset might be able to parse the data for bicycle commuters.
Finally, there are other unique aspects in the bill that extended the qualified transportation fringe benefit to bicycle commuters. For example, the bicycle benefit can not be combined with transit, commuter highway vehicle and parking benefits. Also, unlike other Section 132(f) qualified transportation fringe benefits, the $20 maximum tax free amount for bicycle commuters is not indexed for inflation.