Here's the Silicon Valley forecast for Bike to Work Day 2008.
In a nutshell: sunny, hot, and with elevated ozone levels. The "USG" in the graphic means "Unsafe for Sensitive Groups." I still encourage you to ride your bicycle as fewer cars means less ozone, but stay hydrated, take it easy if you're not heat acclimated and avoid routes with heavy traffic if that's at all possible in the South Bay.
Read the complete post at http://feeds.feedburner.com/~r/Cyclelicious/~3/290068019/bike-to-work-day-weather-and-pollution.html