My tests were clearly unscientific but I tried to be consistent.
I tested 4 laptops: a ThinkPad T42, a Dell D410, an Asus Eee PC 1000H, and a ThinkPad T61.
I ran each through 4 scenarios. First was a Steady state. XP was booted and "idle" as I wasn't intentionally running anything. I made no attempt to stop background tasks. Next, I started a search of the hard drive for a character string in a file name that would be unlikely to be found. During this I subjectively recorded the Search value and the Peak value. Lastly, I put each system in Standby.
The LCD was powered on and the battery was fully charged in all tests.
The Kill-A-Watt only recorded whole Watts so there is probably an issue with resolution in the Standby readings. It read 1 Watt when nothing was plugged into it.
Nevertheless, there are some pretty interesting results:
The Asus Standby effectively read no power draw but that can't be accurate. This is likely an issue with the resolution mentioned earlier.