That's not really it.
If you look on pg 7 or 8 of the 3910 macroblocking thread, there are pictures posted by vetboy and rider that show it perfectly.
Basically, a player that has the "macroblocking problem", exagerates a specific type of MPEG2 compression artifact.
All players have this to some degree, and there are DVDs out there that are mastered with more compression that are more susceptible, but for some reason, the 3910 and 5900 tend to exhbit this "problem" more. It is very specific to certain displays and DVDs.