Although I'd read about the battery drain increase with an SD card in, I recently bought an SD card. It's only 2 GB and I don't notice a drastic increase in battery drain. I do notice it goes to 3/4 quicker than it used to, but I went for almost two weeks without charging it, and using it daily, and it was still at about 3/4 and hadn't gone down from there. That's acceptable to me, as I generally hook it up to the computer at least once a week to download new library books anyway.
It sounds like you put the card in before charging it again, so it could be it was just ready to get down to 3/4 soon and the card just put it over the top? You could always try a test of full charge and count the hours of use until it goes to 3/4 and then charge it up again and try with the sd card. I, for one, would be very interested in your results!
I'm just too busy (lazy?) to try it myself.
