I have to admit that I’m a bit of a “silver bug.” I became enamored with the metal when I was 16 years old. That was the year that OPEC  first jacked up the price of oil. Silver sold for only $3.14/oz. then (per Kitco.) A more accurate way of saying it is that one dollar equaled about one-third of an ounce of silver back then.
Today one dollar will only buy about 1/13 of an ounce of silver (silver at $13/oz.) This implies that the dollar has lost about 75% of it’s purchasing power over the last 36 years, or that it will buy only about one-fourth as much of anything.
Silver has been used as a common, and trusted, medium of exchange (money) for thousands of years. The Greeks coined the silver drachma (about the size of a dime); the Romans coined the silver denarius; and the Americans coined the silver dime, for the first time in 1794 and for the following 170 years, off and on. From 1873 until 1964 the amount of silver in a dime stayed the same. Then suddenly, in 1965, it went to zero.
In 1964 a single shiny new silver dime would buy a 12-ounce bottle of Coke from a vending machine. Three dimes would buy a gallon of gasoline, or a pound of bacon.
In 2009 the same (not quite as shiny) 1964 silver dime will still buy a 12-ounce bottle of Coke from a vending machine, and three 1964 dimes will still buy a gallon of gasoline (with a little change to spare!) Of course you’ll have to sell it first for those nasty Federal Reserve Notes.
The official price of silver is posted on the Kitco web site.  Lately the official price of silver has been low compared to what silver has been selling for on eBay. The eBay prices are typically about 20% higher than the official Kitco “bid” price. The eBay prices are the real, market-driven, prices people are willing to pay. For reference the “melt” value of a silver coin is the amount that its silver content is worth based on the official price of silver.
With that said, these are the typical values (in Federal Reserve Notes) of common US “junk” silver coins as of July, 2009:
Pre-1965 dime $0 .94 “melt” or about $1.13 on eBay
Roll of pre-1965 dimes $ 47.00 “melt” or about $ 56.40 on eBay
Pre-1965 quarter $2.35 “melt” or about $2.82 on eBay
Roll of pre-1965 quarters $ 94.00 “melt” or about $ 112.80 on eBay
Pre-1965 half dollar $4.70 “melt” or about $5.64 on eBay
Roll of pre-1965 half dollars $ 94.00 “melt” or about $ 112.80 on eBay
Pre-1936 silver dollar $10.05 “melt” or about $13 – $14 each on eBay
These prices assume that the coins are non-collectible common dates. In the case of silver dollars there is always a $3 – $4 premium over “melt” because of the numismatic value of the coins. This is for noticeably worn silver dollars. Nicer silver dollars command considerably higher prices.
If the price of silver falls to $ 12/ounce then simply multiply the prices listed by 12 and divide the result by 13. If the price of silver rises to $ 14/ounce then multiply the prices by 14 and divide the result by 13.
I recommend that every serious survival-minded person accumulate and keep a minimum of $ 100 (face value) of pre-1965 silver on hand. Next to bullets, food, and gasoline this is going to be the most important commodity one can accumulate.
JWR Adds: Here is some additional useful data for calculating the bullion value of circulated US silver coins:
90% silver bags ($1,000 face value in 1964 or earlier dimes, quarters and half-dollars) contain approximately 715 ounces of silver
40% silver half-dollar bags ($1,000 face value in 1965 to 1970 mint date half-dollars) contain approximately 296 ounces of silver