Have a T61 with max resolution 1920X1200, Nvidia 140 GPU. OS is Vista Ultimate.. Screen is very bright
Also have a T61P with max resolution of 1920X1200 Nvidia FX 570M GPU. OS is Windows 7 Ultimate. Screen is much dimmer.

1.Does the fact that I have higher resolution on the T61P account for the dimmer screen?
2. Does a dimmer screen mean the backlight is bad on the T61P.?

Have seen ads on Ebay for new backlights. Looks like a real fun time to install one.
Screen is not so dim that i can't read it but it is significantly less bright than the T61 with lower resolution.
Also if i change resolution on the T61P the actual screen size shrinks vertically and/or horizontally.

What gives?

A relatively easy way to find out the (hardware) differences, would be to swap the complete lids on those machines.
One could be LG, the other Samsung.
Or one could have a dying CCFL/backlight or a dying inverter.

But, there's Fn/Home and Fn/End to change the brightness.
In the BIOS/Config/LCD settings you can change the brightness from Normal to High.
And finally, there's Power management that can set/change brightness.
Match all those settings in both machines, then compare again.

LCDs are NOT meant to run with a resolution other than their native one!

Thanks for the info. I had changed the settings in Power Management but not in BIOS. Changed from normal to high and rebooted. Now the screen looks as bright as the one on the T61 with WXGA.
Appreciate your help.