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. 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.
For example, say I have $10,000 in deposits, all of which I've loaned out. I have a $3,000 reserve. Now, I get $8,000 called in, which I can't pay. What I do is I issue $8,000 in interest-bearing notes at double the prevailing interest rates and use them to pay back my customers. My customers will be happy because they can sell these notes for more than the $8,000 they're asking for. As the loans get paid off, and using my reserve, I can pay off the notes. Nobody gets hurt but me.
Runs aren't really a problem. Of course, such systems can still collapse due to bad loans or insufficient capitalization. Generally, a bank has an equity crisis that ends with a run and people think it's the run that killed the bank. But that's just not true. A run can destroy a healthy bank but it can't hurt those who loaned the bank money.
This probably has nothing to do with Ripple though.