Intel Speedstep throttles back the cpu speed to match demand and while running battery power.
Don't have a T60 in front of me but that's probably what you're seeing.
You can go into BIOS and turn it off.
1.66 was the current CPU speed when the software read it.
t2500 @ 2.0 GHz means it's a T2500 CPU with max speed of 2.0 GHz. Doesn't mean it is currently running at that speed.
A couple of overclocker's tools for monitor CPU, temps and other info is CPUZ and HWMonitor. Both available from the good folks at cpuid. HWMonitor is really useful for displaying battery capacities.