Please uninstall ATK. You do not need it. Here's a good post with all the links to why task killers aren't needed (unless you have a rouge app):
http://androidforums.com/android-applications/214451-task-killer-do-i-need-one.html#post1852258
Really strange about your battery life
since all of the reviews raved on this.
These are just trouble shooting steps for when you have battery issues:
1) What apps have you downloaded so far, it might be that you have a 'rouge' app. Look at manage application and show which apps are constantly 'running'. Dial #*#*4636*#*# to get more detailed on your battery life and to see what app is active and for how long. Narrow the 'bad app down' and uninstall them. Test to see if your battery's life has improved.
2) If you are unable to detect what app is at fault, do a factory reset and use the phone with no apps install for a day to see how the battery is. If you have good battery by the end of the day with 'normal' use, then it was a rouge app you downloaded. Uninstall each of your apps one at a time and test each one to see what app is giving you the bad battery life. If your battery life is still bad
3) Take the phone back to the AT&T store and get a replacement battery. If even with the replacement battery, the phone battery's life is still bad then it might be a stock app.
4) Take the phone back to AT&T and get a replacement phone.
Hope this helps and that you get this battery issue fix
-Roze-