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Old thread, but assuming it has a typical cell radio algorithm, what it'll do when out of range is increase wireless power in steps until it gets to max, wait a fixed amount of time, then go into a sleep/wake pattern where it will check for wireless every so often. You definitely see increased power loss in this state over a normal found connection, as the cell radio will decrease its power from full based on the strength of the connection.
This is normal behavior--cell phones drain faster out of range, too. Luckily, the Kindle allows you to turn off the cell radio with a hardware switch, after which it isn't an issue. I keep mine off nearly all the time, turning it on when only I need it, plus once in the morning to get my paper (which arrives within seconds--they must have a wake routine that contacts Amazon upon turning on wireless). My battery life is phenomenal.
The other thing that will drain batteries is indexing, which happens within sleep mode (i.e. it doesn't really go to sleep fully, but stays awake to index while showing you the screen). I suspect it also keeps the Kindle from decreasing CPU/power usage between page turns. If you load a bunch of new books, you'll see battery drop.
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