People have requested GPS enabling / disabling within their audio profiles, but unfortunately Android doesn’t allow developers to have access to it. From what I can gather from forum posts, allowing GPS access raises security and privacy issues (i.e. secretly tracking phone position), so they’ve taken the really conservative stance of denying developers access to that part of the phone.
How about those quick toggle widgets on the Android market? Or the Power Control Widget?
As far as I can tell, those apps use undocumented features of the Android API, and can not be guaranteed to work in the future. You can see in the comments of some of those apps – as people upgrade their version of Android, the widget/app breaks frequently as well. So any GPS functionality will be pretty fragile.
So sorry, automatically enabling / disabling GPS is dead in the water for now.