The two "dimples" in the T61 are not opened, and there is no mic directly underneath it or on top of the Mg rollcage. It looks like it is just an indicator where the internal mic is approximately located, just like those VGA, headphone symbols.
If you look underneath the chassis, I think the silver thing on the motherboard is the mic. with a small opening in the frame above it.
Not sure whether the only way the sound wave can reach it is to travel through the gaps in the Trackpad's mouse buttons or not. It doesn't sounds too great. Recordded 6 inches in front of a 220W Sony SS-U421AV speaker off CDP-C221 CD+TA-AV421 amp. with Audacity 1.3beta in 44kHz, compare to original CD in Adobe Audition 2.0. Not too great above 2KHz already, almost no response >7KHz at all.