I have a few ThinkPads that have the rock when the lid is open and when moving the laptop the LCD lid would rock back and forth, this problem is not just for ThinkPads but even HP and other brands. My 600X, T40, R50 and X40 are still solid, but for my i series, R31, R40 and some others they rock. Some hinges have screws to tighten but some dont. Most if not all HPs I got in did the rock with their LCD lids.

Odd none of my Thinkpads have that issue (don't have an X series). Quite a few other brands have had that issue, generally I just take them apart and tighten all the screws or replace the screws. Could be worse, apples old laptops had exploding hinges.