T410 with I7-620QM and I've noted that the Intel Turbo Boost is not working when the battery is out of the system and I'm only running on AC. (I took the battery out as a conservation measure)
If you are running with only the 65W power adaptor and no battery, Turbo Boost is disabled.
With the 90W power adaptor and no battery, Turbo Boost still runs.
Due to the excellent battery charging on a Thinkpad, leaving it is generally an acceptable practice as it only charges when it drops below around 90-95%. I.e. typicallly it is never charging the battery. You gain the benefit of not having problems during a power outage too!