You can use Ripple to do exactly what you describe.
Yes, I guess so but perhaps I'm not understanding Ripple very well.
It all seems very general and a bit circular.
Let's suppose I have GBP in a bank account. I want to trade them for bitcoins.
I can issue an IOU for my GBP funds via Ripple.
jbloggs can issue an IOU for his bitcoins via Ripple.
That's great, now we can exchange IOUs via Ripple.
I now have jbloggs's IOU for bitcoins, and jbloggs has my IOU for my GBP funds.
So now I have GBP in my bank account (which I've promised to jbloggs via Ripple) and jbloggs has bitcoins (which he's promised to me via Ripple).
But now what do we do? We seem to mostly be back at square one!
I've got GBP credits on Ripple, I sold some BTC at Bitstamp then used the funds from that to issue GBP Bitstamp IOU credits. You can use the p2p exchange built into Ripple to trade any gateways (like Bitstamp) IOU's. As the Gateways IOU's are trusted. Tho I can't personally send anyone the credit IOU's until the other party gives me trust for at least that amount but you can trade them on its inbuilt exchange easily.