Personally, I think eventually community credit will be Ripple's killer app. But for the risk averse, you definitely only want to extend credit to a Nexus where you have a "withdraw on demand" agreement.
If you're a little more adventurous, you can extend credit to other nexuses because you know others have withdraw on demand agreements with them and the risk is therefore very low. Arguably, the risk is lower if you diversify.
Ultimately, if people use the system sanely, I don't think redemption will be a big deal in the sense of one party having to chase another party to get them to redeem. If I extend $50 in credit to you and you use it up, you now have no access to my credit network. And I now have full access to yours as I hold $50 in your IOUs. Assuming you're reasonably well-connected, I can unload those IOUs in my natural course of making payments. Assuming I'm reasonably well-connected, you'll want to get that credit back by repaying me. And, of course, you can always settle in the system by pushing any IOU I accept to me, so you don't need to bother me to exchange cash in person.
Essentially, extend credit to entities whose IOUs you *want* to hold or to people you want to help out by allowing them to owe you money. I think some social adjustments will be needed for this to work, which is one of the reasons I say it will be big "eventually".
For now, we'll probably be telling people to only extend credit to their nexus.