Ripple "allows" anything of value to be traded.  Ripple doesn't put value on things, you and I do.  In order to make a direct trade for Bitstamp.BTC, there would have to be liquidty or someone willing to accept them, at whatever the market deemed was a fair price.  If there was liquidity from Gox.btc to XRP, you could have also made this move and from XRP gone into any currency.



Again, Ripple doesn't put a value on anything.  You're saying (or whoever in your example) is saying they are equal. That's if you're talking about rippling through someone.  An option now exists in the client that defaults to "rippling off" and you have to actually enable rippling for that to occur automatically.

For direct trades...its similar to Bitcoinbuilder.  Some people have taken a risk on GoxBTC and maybe it will pay off, maybe it won't.  It would be the same in Ripple.  The IOU's will likely retain some value and if someone wants to buy them, they certainly can.  They assume that risk, just like the holders of Gox.BTC.  It's just with Ripple, you don't need a central entity like BitcoinBuilder to make these trades.  It all comes down to what you and I deem valuable and what you and I deem are fair prices to facilitate that trade.  Ripple just gives you and I a distributed exchange to conduct business, while eliminating the need for central clearing houses and centralized exchanges.