is a feature of the newer ThinkPads, I don't know if this is a T series only feature. This thing is featured in some of the ThinkPad ads on TV - the one where one guy drops the ThinkPad of his co-worker and don't loose any data because of this thing - a silly ad. Anyway this feature is software based and therefore you need a driver - which is available for Linux since mid 2005 (if I read the website correctly). The hdaps hardware is basically just a tilt and motion sensor, where a piece of software needs to monitor the sensor and park the disk if anything strange is detected.
The Linux hdaps driver of course can do much more then motion/tilt detection. It has a build in mouse device so you can abuse the sensor for stuff like playing pinball :-) and there are other fun applications for testing the sensor.
Anyway I started writing a Gkrellm plugin for it which works quite good. But from time to time I get a Kernel ups in the ibm_acpi module (which works find without hdaps), so I decided to not use hdaps for now. But I really like this thing and I look forward to get a ibm_acpi compatible version (if this is the real problem). Any one who ones a ThinkPad should check this out.