So... you've only come into contact with three XOOM units and all three units do this?
We have three XOOMs (my wife, my son, and I). It's not happening to ours... luckily.
This seems like it's a display maker issue. By design, the displays are supposed to withstand more strain then the glass will exhibit under a reasonable/normal stress.
Or it could be a glass maker issue, where the glass strain is greater than design specs indicate it should be.
Also, make sure you're not abusing the tablet. These things aren't steel plates after all. The majority of the rigidity of all tablets (no exceptions) comes primarily from the display and glass. If these are stressed beyond reasonable limits, especially during transport, all bets are off. People may be surprised to find out that notebook computers can take more stress than tablets... anybody's tablets/pads.
BTW, I'm using the engineering definitions of stress and strain, not colloquial.