Even if the netbook is shut down or hibernated! This includes all the Z series I have tested so far: Z540 (HP Slate 500), Z530 and Z520 (UMID M1). Initially I thought it was a motherboard design flaw, but all netbooks that use these CPU have reported such battery loss, including Sony P.
How much battery loss? About 3-5% overnight. Seems not a big deal, but you should be aware of this flaw. For example, you bring such a netbook to a long trip, after a week or two, you finally have a chance to use it but only find an empty battery.
If the battery is removable, you'd better turn off the netbook or hibernate it and then remove the battery. For the HP Slate 500, the battery is not removable, I don't know how to deal with this problem. So far the good news is the battery always drains just 3% during hibernation, for a period of 8, 15 or 24 hours. I have not tested it longer (see update below).
The earlier Intel netbook CPU, such as N270 (ASUS eeePC 900a), does not have this flaw, but they have another one: battery drains way too fast during sleep mode. So the sleep mode is useless and you have to use hibernation always.
Newer Intel netbook CPU, such as N450, have both flaws sorted out, but they are garbage because the GMA 3150 GPU that usually companies with them no longer do 1080p. This is why you no longer see netbooks in stores; who want to carry something that cannot do HD these days? Not sure about the new Z670 and Z690, but who cares as they are slower than Z540! Intel, why would a company make its products worse and worse?
AMD netbook CPU, C-50 (Acer 722 and W500) and Z-01 (MSI 110W), do not have these issues.
[update] 2/25/2012: Let the HP Slate 500 hibernated for 3 days (72 hours), battery level dropped from 86% down to 77%, that's 3% drainage per day.