"Trust" in Ripple means:
"I would accept a check of up to __ units of currency __ from issuer __."
When you cash this check and if it is actually still "good" is your problem. If you accept checks from 2 different issuers, you can trade between them (e.g. I can offer to pay you 0.99 BTC for a 1 BTC "Bitcoin Code" from MtGox), which is how Ripple does path finding.
TradeFortress for example wrote himself a check of 1 billion or so BTC and then claimed this is a flaw in the system. It isn't - it's exactly how it is supposed to work. There is nobody stopping you from taking a blank check and writing "1 googol USD" on it either. You might have a hard time paying with it though probably.
Except this is only good hypothetically, and doesn't pass a reality check.
You can't really trade IOU between two issuers because they aren't on the same terms even if the value of the debt may be the same.
For example, there are differences in withdrawal limits that are not reflected in the Ripple system. One issuer may allow for withdraw of up to $1000 per day, and the other may allow up to $50,000. If you receive IOU from the first issuer, it will take you 50 days to recover your debt instead of 1 day from the latter.
In addition, how you withdraw your funds between the issuers are also different. Some allows for withdrawal to Dwolla, some allows for inter bank wire within only Europe or China only. There is no use getting a IOU if there is no way of recovering the debt.
I suggest Ripple proponent to read up on how how the current Bank Wire System works (
http://money.stackexchange.com/questions/21775/how-do-wire-transfers-get-settled is a good start).
Then re-look at the Ripple system again, and see if it can really achieve true decentralization operation when Fiat money is in the equation.
Bitcoin is brilliant because it explains its solution in a non-obscure manner. It is solving a "Double Spend" issue using decentralized timestamp system -- easy to understand.
All Ripple does so far is making lots of assumptions and hypothetical examples (that show a lack of understanding of the actual financial system) without articulating what exactly they want to solve, and the value its solution to the various players in the eco-system.