The Kindle is effectively "off" in standby mode. You can't send books to it, unless you use a USB cable, in which case it boots up in USB mass storage mode. Or is wifi still on on the Kindle 3?
Funny, I send books to my Kindle wirelessly all the time in standby mode, and when I wake it, they're already there. Perhaps you're confusing standby (displaying an image, from either the ~4sec power button push or inactivity timeout) with the off setting (~15sec power button push).
If the above is true, I assume Standby merely prevents accidental key presses and does not help maintain the battery life, right? After all, you can still send books to your Kindle while it is in Standby mode.
*Right or wrong, can anyone confirm this?