Between the hills, fog and traffic, bicycling provides a fast, exciting, and warm (Cold?! Ride faster!) way to get around SF!
Get a bike with a granny gear, keep your eyes open, follow traffic laws... and always-always-always watch for people popping open their driver-side doors into the bike lane. Otherwise, the Muni and F-Market rails can be treacherous in addition to anything metal and/or painted on moist (or even foggy) days.
For those tired of the hills, the Embarcadero offers a beautiful flat stretch from AT&T Park to the Maritime Museum, and beyond that only a small hill through Fort Mason to the Golden Gate Bridge.
Bike politics is hot - the SFBC is highly active and successful - building bike lanes, promoting bike commuting, hosting events, etc - and counts most local politicians, including former Mayor Gavin Newsom, as supporters. The Critical Mass ride is worth checking out on the last friday of the month.