HTC has an alternative API that supports this, however it’s only on HTC Sense devices, and as of Gingerbread they’ve changed the permissions so it’s only for their Flashlight app, not third party ones (unless you use root).

But on 2.2 HTC devices you can use it by writing a string to /sys/devices/platform/flashlight.0/leds/flashlight/brightness. This controls if the LED is on and how bright it is. For maximum brightness write "128\n", half brightness write "64\n". Easy to test from adb shell:

echo "128" > /sys/devices/platform/flashlight.0/leds/flashlight/brightness

And finally turn it off by writing "0\n" there.