I suspect it's simply how much you're using the screen.
I've had my Kindle 3 for 9 days now, and after a full charge I did within the first day or so of getting it, it's a bit less than half charged. I've used the wifi/3G on and off, plugged it into the PC a number of times to add and remove files, and flipped the sleep button countless times to screw around with the screen savers. All of the adding and removing of books makes it do the indexing, which clearly hurts the battery. I've also been using it a lot for actual reading, so I'm probably doing much more than the average amount of page turns on it per day.
So while at best it looks like I'll get about 2 weeks on my first full charge, I think Amazon's estimate of a month is based on casual use.
It doesn't surprise me that for you the battery level dipped after 24 hours and 5000 locations read. That seems like a lot of usage for one day.
Of course I haven't had my Kindle for long, so I'm not the best judge of this.
-HM