Reflash your NIT and leave it always on. Usually the battery drain is gone after a reflash. Better wait for Diablo to reflash.

Turning it off after inactivity is no solution for this problem. I wouldn't even call it a workaround.

My battery life when I got the IT was awful, but now it is fine.
I removed the active applets on the desktop (wireless monitors etc), turned the wifi to never automatically search and timeout after 10 minutes.
It doesn't affect anything in the use of it, but now the battery lasts quite a few days of non use, or a few hours of full use. Before that it never made the day.

To directly answer the OP: I know of no way to do that; if you really, seriously, are determined, I bet you could hash something up with dbus-scripts, watching for the screen to dim.

But I'm with GA et al.; you don't need that, you need to find out what's slaughtering your battery.

Candidates to check:

• metalayer-crawler: automatically background crawling on this class of device is just plain wrong, imho. It impairs battery and usability (even when it is working right) if you put in a card with several GiB of stuff; it's known to go berserk when it hits a corrupted filesystem. The built-in media player is pathetic anyway, so nuke this.
• Web browser: sometimes goes mad after going to certain pages (likely Flash-related, but not sure), and sucks 100% CPU for the rest of the session, even when on a clean page. Use a CPU monitor in the statusbar, and close the browser at your earliest convenience after it goes mad.
• RSS reader: I don't use it, but it's widely considered bad for CPU usage.
• Skype: only saw it go nuts once in several months, but you may as well close it if you're not online.
• WiFi searching: scanning for APs is very power-consuming; if you're in marginal coverage, it may drop, re-scan in 10 minutes, and repeat. Turn off auto-connect, if it spends much time in marginal areas; otherwise it's normally not a problem. (But you could anyway...

I never know for certain when I put the device into its case whether the battery will be there when I go to use it again. Turning it off is one way to guarantee it will not have a completely drained battery when I go to use it again. I now lock the screen and put it into offline mode when not in use. This seems to help.