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.
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Crikey, 94 degrees! It was 41 degrees this morning when I rode in. We're looking at maybe 70 today for a high.
The heat is mostly during the afternoon. During the morning rush hour, temperature will still be in around 70F. Evening might be a little toasty, though. Caltrain is an alternative, if it is not too crowded.