Since your physical silver has to be held by a trustee somewhere, you may want to consider just using a stock broker as IRA Custodian and purchasing either an Exchange Traded Fund or a Closed End Mutual Fund. IShares runs the biggest silver ETF. Symbol is SLV. You can buy this just like a stock with low commissions through a discount broker.

Another possibility is Central Fund of Canada, CEF. This has been around for years and holds a combination of gold and silver bullion in an independent bank vault in Canada. Your broker can also buy this and hold the shares in your IRA.

http://us.ishares.com/product_info/fund/overview/SLV.htm

http://www.centralfund.com/

I consider both to be very safe, but if the US Govt keeps getting crazier ideas of how to redistribute wealth in this country, there's no assurance they won't try to get your metal out of the funds and replace it with Treasury Notes or something as the ship starts to go under. The only protection against this is holding the physical metal yourself, but you don't get the tax advantages of an Individual Retirement Account, of course.

For alternative ways to buy and hold the metal, the web site at Kitco.com is invaluable. They have a number of ways to sell your the metal without paying the high premium for bullion coins.

Finally, don't forget that circulated pre-1965 US silver coins are still available. Pretty low premium and small denominations in case you want to actually use the silver for barter transactions as the US currency crumbles. Available in $1, 50c, 25, and 10c denominations and sold through coin dealers (small amounts) or metals brokers (bags of $1000 face).