While ripple might eventually be a great alternative to bank loans, it is still a debt system. Any debt system could collapse when there is a "run on the bank." With ripple, a run on the bank would happen when everyone decided they wanted to call in all their debts from a large borrower. Only then would you know if there was actually enough BTC currency to back the debt. Chances are the answer would be no; debt systems lend themselves to fractional reserves.
If there's a run on the bank, you may be unable to meet immediate demands for called in debt.
But you can simply issue notes worth more than the called in amount [*1]. This will bankrupt *you*, but that's the risk you take when you implement a fractional reserve. Those who loaned you money won't suffer.
This probably has nothing to do with Ripple though.
Do you assume "you" are a central bank in this case?
The "you" is whoever is running a fractional reserve system.
*2: (not up there;) The problem with bank runs and fractional reserve is that banks loan out 10 times or more of what they actually own. When there is a bank run and people demand their deposits back the bank can only return 10% of everybodies funds, thus hurting the lenders alot. Banks in such situations find it quite hard to issue the interest-bearing notes you were talking of, because other banks have the same problems and are trying to find buyers for their own notes as well.
Even if that's true, provided the bank has assets significantly greater than its obligations, it can still pay everyone back eventually with significant interest. People may be inconvenienced by not having the liquidity they expected, but they can't lose their savings. If a bank doesn't have assets that are significantly greater than its obligations, then it's undercapitalized, and the run just exposes that problem.
My point is simply that a run cannot cause depositors to lose their deposits if the bank was healthy prior to the run. Bank runs are a solved problem for everyone but banks -- a run can bankrupt the bank.