OK, I'm new to microprocessors and the arduino platform so please forgive any stupidity 
I think I've killed my nano. There is a chance that I accidentally caused a short circuit while the nano was connected via usb to my laptop, as, well, it no longer works.
When connected to the USB power the tx/rx LEDs flash as usual, then the onboard LED flashes on and off very fast. That's certainly not the program that I was playing with before the problem started (I was playing with serial comms).
I spoke to a helpful chap on the arduino IRC channel and he suggested that perhaps it was the FTDI chip that was broken, as my dmesg output is;
[ 667.514252] usb 1-2: new full speed USB device using uhci_hcd and address 9
[ 667.600298] usb 1-2: device not accepting address 9, error -71
(ubuntu 8.04 on an eee pc 900)
When plugged into a windows machine, I get "device is not functioning correctly" or similar.
Is there anything else I can check before buying another one?
Many thanks 