The GPS isn't on unless an application opens a connection to it. Once all connections are closed the GPS is turned off. In some modes that applications can select, the GPS is powered down between fixes.
On GSM units Google Maps goes into cell tower location mode when in the background (unless IIRC specifically configured to keep the GPS on for Latitude). CDMA units may be different.
The GPS does use a lot of battery power, as does the increased traffic to download maps or other data on demand. Google maps is fairly good at limiting this if it is in the background, Blackberry maps is low impact because it uses vector map data which requires less data.
My other Blackberry is a PlayBook.