in 2.1 the trackball only comes on when the light sensor detects low light. hold your finger over the right 'notification light' (light sensor) and move the trackball around. it should come on. it's a great idea if you think about it.
one serves as a notification light...the other is a sensor. and if you hold your finger over it and move the trackball it comes on. or in darker lit locations... if this doesn't happen for you, well sprint's 2.1 update is a joke - so if you have that...yikes.