That liquidity goes for the XRP part, but the other currencies are tied to the gateway as an IOU afaik. That's why I would think there will be inertia and natural monopolies will emerge, and also because some gateway will be most trusted, like MtGox is.
Yes there can be a most trusted gateway (i.e. the one that manages to attract the largest total amount of deposits across all currencies). But those deposits can be used to exchange for balances at any other gateway. I'll give you an example, with two hypothetical gateways "Bitstamp" and "Foobit", that each issue both BTC and USD. Bitstamp is the big gateway and Foobit is the tiny one.
First recognize that there are 20 order books in this scenario (!!!):
BTC.Bitstamp -> XRP
BTC.Bitstamp -> USD.Bitstamp
BTC.Bitstamp -> BTC.Foobit
BTC.Bitstamp -> USD.Foobit
BTC.Foobit -> XRP
BTC.Foobit -> BTC.Bitstamp
BTC.Foobit -> USD.Bitstamp
BTC.Foobit -> USD.Foobit
USD.Bitstamp -> XRP
USD.Bitstamp -> BTC.Bitstamp
USD.Bitstamp -> BTC.Foobit
USD.Bitstamp -> USD.Foobit
USD.Foobit -> XRP
USD.Foobit -> BTC.Bitstamp
USD.Foobit -> USD.Bitstamp
USD.Foobit -> BTC.Foobit
XRP -> BTC.Bitstamp
XRP -> BTC.Foobit
XRP -> USD.Bitstamp
XRP -> USD.Foobit
Now imagine that someone places BTC up for sale in the BTC.Foobit -> USD.Foobit order book. How can someone who is holding USD.Bitstamp purchase these bitcoins? First recognize that we want to send Bitstamp USD and receive Bitstamp BTC. Now consider one possible path:
USD.Bitstamp -> USD.Foobit -> BTC.Foobit -> BTC.Bitstamp
Ripple will look at these order books to calculate the depth and price:
USD.Bitstamp -> USD.Foobit
USD.Foobit -> BTC.Foobit
BTC.Foobit -> BTC.Bitstamp
It is unlikely that gateways will maintain walls in these books, because it would expose them to counterparty risk. Instead, liquidity providers (I plan on being one) will use software to keep automated bid and ask walls in the appropriate order books. For example, I will accept Bitstamp USD and give you Foobit USD. I will need to charge a small premium, which I will build into the offer. I might give you 1 Foobit USD for every 1.03 Bitstamp USD that you give me.
Repeat the process for each order book and now Ripple can supply you those Bitstamp BTC in exchange for Bitstamp USD, even though the best prices and depth are in a different issuer's order book.
As you can imagine, the number of combinations of order books across all issuers and currencies will explode in number. An alternative is that liquidity providers can maintain bids and asks for issuers currencies in conversion to XRP. The path in the previous example could also be written this way:
USD.Bitstamp -> XRP -> USD.Foobit -> BTC.Foobit -> XRP -> BTC.Bitstamp
or this way:
USD.Bitstamp -> XRP -> USD.Foobit -> XRP -> BTC.Foobit -> XRP -> BTC.Bitstamp
These examples show how XRP can be used as a "bridge" currency. By the way, this is all in the wiki.
Wouldn't this give you IOUs of different issuers? So that to redeem them, you would have to create an account with each issuer.
You don't need an account at each issuer. That's the whole point of Ripple. You deal with a gateway that you trust, to handle deposit and withdrawal, and then you can access everything on the Ripple network no matter who the issuer is.
If you are holding the currency of a gateway that you don't have an account with, Ripple can easily exchange it for something that you would prefer to have by automatically going through order books and "rippling" through people who have extended trust to multiple gateways for the same currency.
This is the distributed exchange Bitcoiners have been dreaming of!