Yes, you are right, in theory, you should get all the same prices from Expedia, Travelocity, ITN, etc - but you don't.

The reason - the data behind those systems are all the same, but the methods they utilize to mine that data varies, especially when looking for the lowest fares. Since there is some many different permutations, how long the engine takes looking at all the different possibilities determines how what kind of prices and flight choices it is going to show.

Also, the underlying systems (Apollo, Worldspan, Sabre) do not all have the same identical airline members in them. Yes, most of the big airlines are in all of them, but some of the smaller niche carriers are not in all of them. (For example, I believe that you can find SouthWest Airlines flights in EasySabre, but not in Expedia).

So, when I book a flight, I tend to check a couple of non-airline sites and have find big differences. Then, once I find the lowest, I then go to whatever site is going to give me the biggest bonus, including the airline's own site (ie: to get extra freq. flyer miles, discounts, etc).