Wow. I just had to read through about 50 fricken replies that just flamed the OP. What the hell?
I've had the same issue since I got my phone in December. Mine happens at pretty much anything below 40%. I kept searching and couldn't find anything about it. I'm running 2.2 now and the issue still remains.
Just because half of you sit around at home next to your chargers doesn't mean we all do.
Thanks for posting this, tassadar898.