Some of the new Thinkpads (T400/T500 series) are being manufactured with lighter keyboards that contain a lot of flex, compared to older Thinkpad's and their rock-solid keyboards. Lenovo finally figured out the problem and are offering a keyboard replacement program, where you let them know your laptop model, and they send you back an older-style (T61) keyboard, which you slap in, and apparently that fixes the problem.

As of yesterday (March 9th), the new laptops are supposed to be manufactured with the new, lighter keyboards, but still maintain the solid feel.

They're stopping the keyboard replacement program as of March 31st, as the manufacturing problem is supposedly fixed.

More information on the Lenovo forum