I've created a little app that lets you set the touch key brightness, but most significantly, restores your settings on boot.
It's 99p and you can find it in the Market.
Just to give some background for those who like to hack around... the backlight value is read / written at '/sys/devices/platform/star_touch_led/wled'. The normal values made available are 00-20, you can program higher but it's probably not a good idea!
The default level can also be changed in the kernel.