Also, under the assumption that you trust your friends, there are still other ways to conduct the transaction, just wire transfer money to your friend using online banking, and ask someone whom you both trust to escrow the bitcoins/audit your wire transfer, such a process is formalizable as well.
With ZeroReserve you can trade with strangers by only having to trust your friends.
Further, if you can have a cryptographic proof of transfer, why the need for IOUs?
There's always to sides of a transaction. It's difficult to computationally enforce transfer of non cryptographic currencies and assets - but you can easily transfer IOUs with people you trust.
Effectively, the Ripple trust path does this by going through someone you both trust, so it's not really different from having that someone escrowing your bitcoins, and at the same time audits your fiat transfer, isn't it?
You don't need to really enforce the bank transfer, you just need to verify that it has happened, before you release the escrow.