Hello,
Just got the same problem, with an old duemilanove.
It start doing so after uploading a buggy sketch that I try to debug.
The on-board led flash fast and reset button is no use.
but I could reflash the board with pressing the upload button just after the usb cable was plugged. I's a bit tricky to achieve cause too early the com does not exist, and too late the bootloader started up the buggy sketch.
Maybe it could help.