I don't know what you mean by "Bitcoin IOUs", but I believe that Bitcoin FRB is a certainty if Bitcoin becomes widely adopted. People will want loans, and banks will pay interest to depositors, and then loan out their money (assuming it is all legal).
Any time you do not personally control the private key you are accepting a "Bitcoin IOU" because you have deposited the BTC with someone else and they now owe you the BTC - and they control the private key.
Examples: Coinbase, Localbitcoins, any Bitcoin bank in the future.
By making a deposit you have accepted a Bitcoin IOU for your Bitcoins.
Now they can lend them out, FRB is born.
For FRB to really get going people need to accept paper bills or checks or wire transfers or deposit acounts as Bitcoins - which they are not. They are "Bitcoin IOUs" or transfers of "Bitcoin IOUs".
I also agree with you that it is certain to happen because people in general are scared to carry and possibly lose their cash/Bitcoins so they will "leave that to professionals" - certain, just a matter of time and wider adoption.