In the flicker discussion group Gary Weinheimer gives us a great example of the difference between using a single strobe (the photo above) and using 3 strobes (the photo below).
The point I want to raise is that after you master the basics like straight walls, correcting lens distortion and getting white balance correct, the single most important factor that contributes to the quality of an interior image is what approach you use for lighting.
There seems to be a natural progression in real estate photographers approach to lighting:
Each one of these lighting approaches give successively better results. An to my eye multiple strobes give the best results. I was satisfied with a single on-camera strobe until I saw the results that Gary, Scott, Aaron and M. James were getting with multiple strobes.
Make no mistake this technique is more difficult to master. There is more equipment to carry but to me carrying a little more equipment and learning these techniques is worth the effort.
I know that some will argue that one can get good results by using HDR techniques via software like Photomatix. But I see very few interior shots processed with Photomatix that do not have strange and noticeable artifacts.
So to summarize, all of these lighting approaches work well in many situations but I believe that being able to take complete control of the interior lighting by using multiple strobes gives the best results in the widest number of situations.
7/6/07 Update: I just noticed that David Hobby over at the Strobist has a better description than I've done of this lighting progression. David calls it "The Lighting Journey" and describes seven levels that photographers pass through as they strive to improve their lighting technique. David describes this phenomena beautifully.