applications, applications, applications
(including widgets, tweaks etc.)

Start digging. User search. Loads of threads about how to find misbehaving apps. If this is too hard and you do not have time then make backup to desktop pc of N900 and flash emmc+rootfs (from maemo.org wiki more info) and test if factory default device without any apps lasts longer than 7 hours. If not then you have HW defect and you can send it to Nokia*.

* of course everything depends on what you do with device but if it lasted before update over 7 hours with certain usage scenario it should last also with newest pr 1.3.