The battery charge indicator on both my Rebel and 40D have a funny habit. They tend to always indicate a full charge. After a lot of use, when I'd expect the charge to be depleted, and the indicator still shows a full charge, I disconnect the battery grip from the camera and reconnect it. Then the indicator changes it's mind and shows the battery on its last bar of charge.
Mine always seems to appear as a full charge when I look. Then one day it is on the last bar. I know a single charge lasts a long time on the 40D. Sometimes I go weeks before changing batteries. The meter seems to react the same whether the grip is on or not.
judging by the flashing lights on my charger, i think that the when the camera battery indicator is on the last bar, that is half the charge left / gone.
i have never experienced slippys problem though (i don't use a grip either)
My 40D seems to show as a full charge until I have 20-40 shots left (dependent on conditions) - so yes, I have the same problem.. The camera can be showing it as fully charged, but if I then put it in the battery charger it will blink to show it is less than 50% charged.
My 40D stays on full for quite a while, then goes to the last bar, where it stays for a long time before flashing to say that it needs changing/charging. It is a recognised problem that has been inferred in magazines, although not specifically for certain Canon's. It has apparently been fixed in the 50D and 5D Mk2 (it was in those reviews I saw mention of it).
