Well, that didn't take too long to work how much of a muppet I am! It turns out, there's nothing wrong. If I'm powering the Core2 off and on, the USB connection does not re-start properly. No matter what code/sketch I upload.
I think is coming down to a driver/PC issue! I should really have looked at this problem using a completely different computer!
Needless to say, it's not specific to this code (above), but a problem elsewhere! DOH!