What is the stock market symbol for Silver futures?

There is none as Silver (a metal) is not a stock from a corporation.

Wow. Like most things today that's technically correct, but useless.

I believe that what you are after are the futures or options symbols.

The symbols are different depending on which commodities exchange you look at. For the current contract, Dec 09, the COMEX ticker is SIZ09.CMX. Each contract has an expiration month, but not all months have a contract. Each month has a letter associated with it, not necessarily related to any of the letters in the name of the month. The letter for Dec is Z, for Jan it is F, Feb is G, May is K and so on. So the ticker for May 2010 silver is SIK10.CMX

You can find quotes here:

http:/wwwzperiodzcmegroupzperiodzcom/trading/metals/precious/silverzperiodzhtml

Silver options (options on futures contracts) have similar ticker criteria.