I had this problem in LR2 and now in LR3. When I click on the adjustment brush it does not function. The gradient, red eye and spot remover work just fine.
This is a problem on my windows XP desktop, which has both a mouse and a Wacom tablet. It works fine on my windows XP laptop loaded from my second license install of LR3, using either my mouse or the Wacom.
Because the adjustment brush didn't work in LR2 either, I'm wondering if this is hardware related?