How much water should I expect to be left in a dishwasher?
I had exactly the same issue with my dishwasher.  Standing water at the bottom after the wash cycle almost every time, yet the dishwasher appeared to be draining correctly.

Nevertheless, the first thing to verify is that the dishwasher is indeed draining properly.  Start a cycle to partially fill the washer, then manually engage the drain cycle.  Does the water drain quickly and completely?   If you're so inclined, you can (carefully) leave the door open while the dishwasher is draining and hold down the door lock mechanism with a small screw driver so that you can watch the process.  Another option is to disconnect the drain from your disposal and place the end into a bucket to observe how the water is draining.  It's also a good opportunity to clean out any sludge that may have formed in drain hose.

If the draining appears to be functioning normally (as it was in my case) the next step is to verify the filter, food chopper blade and mesh grate that is in front of the sump pump/pump inlet housing is not clogged - this is slightly more involved, but still fairly easy to do.  Here's a video to give you the general idea of the process:  http://www.youtube.com/watch?v=lRDbTgBOtIE.  This is not for your model specifically, but it should be fairly close.  

In my case, the mesh grate was almost 100% clogged with debris and hard water deposits, and the food chopper blade had a piece of string wrapped around it that kept it from spinning freely (no idea how that got there).  After soaking the grate in a vinegar solution, cleaning the it with a toothpick and unwrapping the string from the blade, I haven't had any more issues with water remaining in the dishwasher.  

This may not fix your problem, but it's fairly easy to do yourself and may just save you a service call.  Good luck.