Todd has the most likely culprit. What he is talking about is that thing that sits next to your faucet on the sink that makes gurgling sounds when your dishwasher is draining. If this is blocked, then no air is getting through allowing the dishwasher to drain properly. This can easily clog because under the cover, which just pulls straight up or may have a couple threads and need a turn, is a "U" shaped copper tube.
...or you can do like it did and install a new garbage disposal and forget to knock out that little plug where the dishwasher drain hose connects to the disposal. It's quite fun when the dishwasher drains for the first time and the water comes up thru the aerator.:b
Yeah, I'm not a plumber.
I did the same thing Ron did. I put a new garbage disposal in, and failed to knock out the plug in the connection for the dishwasher hose. I never would have discovered my error, except that a friend at work was having the same problem. He called a plumber to diagnose his problem. I got the advice from him for free Since then I have successfully passed on the information to two other not-so-plumberish people.