Did you ever find a solution? Seems I have the same issue on my T400...
I have a r61, type 7743, ag6.
It had been working fine until now but suddenly this morning it wont startup at all. On pressing the power button all lights flash normally for 2-3 seconds, but, just before the display should turn on, it turns off completely. Even on pressing power key again, it wont respond in any way unless i remove the battery and replug the power supply.
I have already checked the fan which is rotatng, reseated the RAM but nothing works. This had happened earlier also but then it was intermittent and would correct automatically. Also when it did boot it was fine until i shut down the laptop properly. But this time, it is just not booting up.
Kindly suggest some solution, I really want to avoid a Mobo replacement.