The official story: There is no Halloween celebration in the Castro tonight. The official San Francisco celebration is located at "Lot A and Terry Francois Blvd." (near the ballpark). The details of this event can be found at www.sfhalloweenfestival.com (live entertainment including Donna Sachet and Martha Wash, pie eating contest, "Summer of Love" stage, haunted house, and Much, Much More!).
According to the site, nohalloweeninthecastro.com, (aka "Home for Halloween") "There will be NO Halloween celebration in the Castro in 2008.
"As was the case last year, there will be NO Halloween celebration in the Castro in 2008 and the streets will NOT be closed. Traffic will flow normally. Bars and restaurants will be open for business."
The site even provides several videos to encourage people to stay away from the Castro:
Home for Halloween
According to MUNI, "There are no service changes planned for Halloween. There will be no Halloween celebration in the Castro in 2008. San Francisco Police Officers will monitor pedestrian and automobile traffic for any necessary public safety street closures." (However, they also remind everyone that today is also Critical Mass day.)
According to the signs posted on Castro Street and on Market at 17th, there is no parking permitted from 3 pm until 4 am. Which, to me, sounds like they intend to shut the street down anyway.
Of course, this might become a non-issue if all this rain dampens spirits sufficiently....