This is not entirely correct, the most active (over-the-counter) market is right here on Bitcointalk forum. And I am the most active dealer of this market, with more than million XRP going through my hands just last week.
If someone wants to trade for IOUs inside Ripple, there are currently 4 active markets:
BTC/XRP bitstamp (most liquid)
BTC/XRP weex (bigger spreads, but better deals sometimes)
USD/XRP bitstamp
USD/BTC bitstamp
Trades happened in all these markets, and USD/BTC bitstamp is following MtGox price movements to a reasonable extent, even though spreads are not that good.
All in all, Ripple gateway order books have good potential to develop into full-blown currency exchanges over time. They are just not there yet, the volume is minuscule and you're better off trading OTC here on Bitcointalk at the moment.
How do we know that trades have happened in all these markets? I haven't been able to find a list of trades consummated.