hiduino:
I recall similar problems like this with early versions of the IDE v1.5 when it first came out. Make sure you are using the latest IDE v1.5.7.
Thanks for the reply. I was using 1.5.6, now I tried 1.5.7, it doesn't work either. I just uploaded the blink example, the on-board led didn't work after power off for a while. Reset button made it work again as I mentioned in the previous post. Any idea?