For what it's worth I believe the chipset itself is supported but the motherboard does not support software control of the fan - and if the hardware doesn't support it there isn't really anything that can be done.
I installed SpeedFan on my desktop (also has a 965 express chipset like the X300) and Speedfan was able to pick up the fans I have connected to the motherboard. This is why I think the X300 motherboard just simply doesn't support software monitoring/control of the fan.
And if you look on ThinkWiki
the X20xx and X30x models are not listed with all the other Thinkpads that do support software fan control.