I remember reading something about this before, and thought the same thing as richardhenry mentions. It sounds like a gimmick to wow those who don’t know. So did a quick search, a quick summary of what I found.
The Vista feature is called ReadyBoost and it looks like flash memory (like usb drives) balances out the slow transfer speed with faster seek times, so it’s only used in cases where the shorter seek time counters the actual time to move the data. (I’m thinking maybe if you’re recovering lots of tiny files from cache?) Since a hard drive has to physically move to the location of the information, read, and then transfer it looses some or all of the edge it has in how fast it can actually push the data. Also, remember we’re talking times in milliseconds here on all counts.
Wikipedia knows more than I do ;) and The Google turns up lots of results as well.