Saw this posted on Toyota Nation and wanted to pass along the info.

SYSTEM MALFUNCTION
The tire pressure warning system does not function properly under certain cir-
cumstances. In the following cases, the low tire pressure warning light may not come on even if the tire inflation pressure is low, or it may come on when the tire inflation pressure is actually normal.
A lot of snow or ice covers the vehicle, in particular, around the wheels
or wheel housings.
The tires are not equipped with an air pressure sensor.
Snow tires or tire chains are used.
Non- genuine Toyota wheels are used.
The sensor battery is expired.
The system cannot receive radio waves from the air pressure sensor on the
spare tire, depending on where the sensor is located when the spare tire
is mounted on the vehicle. In this case, rotate the spare tire by
about 90°so that the system can receive radio waves

Left out the couple of ludicrous ones inspired by the FAA
Electric devices facilities using similar radio wave frequencies are nearby.
A radio set to similar frequencies is in use.

If the warning light comes on:
Stop your vehicle in a safe place as soon as possible and check that the
inflation pressure of all tires (including the spare tire) is as specified on the
tire and loading information label. (See “Checking tire inflation pressure” on
page 366 in Section 7- 2.) If the warning light comes on even after tire inflation
pressure adjustment, you may have a flat tire. If you have a flat tire, replace
it with the spare tire. For details, see“If you have a flat tire” on page 321 in
Section 4.
The warning light goes off a few minutes after air is put into the deflated tire.
This warning light may turn on due to natural causes such as natural air leaks
or tire pressure changes caused by temperature. In this case, adjusting the tire
pressure will turn off the warning light.

More info here - http://www.toyotanation.com/forum/61...ma-tpms-2.html