For one thing, before we can distinguish between the two sample responses you give we need a defined point in time at which the IOU issuer is obliged to give a response at all (a settlement date).
IOUs (taken as meaning any acknowledgement of debt) don't necessarily EVER have to be redeemed. For example perpetual bonds are never intended to be redeemed - yet are debt.
Thanks for your valuable arguments. You might be right and I think I understand now why theymos said IOU is not a binding contract. But now I feel totally confused. What is debt then, if it is doesn't ever have to be repaid?
Maybe ripple should add a popup when someone tries to extend trust - refusing to do so unless the user ticks a box saying "I have agreed to this issuer's policy on redeeming their IOUs in general and, if applicable, this one in particular".
Yes, that might be useful. I don't yet know how IOUs are being issued in Ripple, but maybe they should have that policy included.