Is only makes sense if you have a number of applications running in the background, such as an active web browser page, JiveTalk, or a gps app.
Press ALT and ESCAPE. Do you see such as those running? You should always see icons in this taskbar for the homescreen, phone, addressbook, BBMessenger, and the browser (just make sure it is not on an active page).
That is likely your problem.
**Edit: and read this: Maximum Battery Life - BlackBerryFAQ