That's the question ACHD (our local highway district) will be trying to answer this week, by doing bike-rider head counts at various strategic locations.
More info HERE.
Since their baseline counts were done in the month of April, it doesn't seem like very high-quality research. But I'm no expert. (I would expect the numbers to be substantially higher in September 2008, than in April 2007, not only because more people are riding bikes on account of high gas prices, but because September is a more solid month weather-wise, than April.)
You local bike riders - if you see an earnest-looking person with a clipboard and a little hand-clicker as you do your riding, go around the block 3 or 4 times to artifically inflate the count! (Of course I'm jesting.)