No, I didn't think of the polyfuse. Thanks for the suggestion, I appreciate it!
And I believe I have lied to you all. I hadn't touched the board after it fried, so this morning when I went in to the morgue, I noticed something I missed yesterday...
I had a lead connected to the barrel of the DC jack as a low-impedance ground for my circuit (pulsing a small detector coil). Today, I noticed that the lead was right next to the USB shield (shield as in the metal jacket around the PCB connector, not shield as in an Arduino shield! Or a hoplite, either!). It is entirely possible that this big, heavy ground wire could have connected with the USB parts. It would have been from the top, not the bottom (I'm paranoid about stuff getting under any of my boards, so I have a non-conductive antistatic foam dam around the board's underside), so the fuse issue may actually be what went wrong...
Now that I've seen that potential hazard, I can't claim to know for sure what the hell happened. Sigh.
I contacted the local vendor, who, like most other vendors I've spoken to this morning, are completely out of stock of Dues. Which is good, I guess - they're obviously going to be tremendously popular! But it makes it frustrating for an Australian developer with only Paypal.
Still, at least knowing about the polyfuse gives me some incentive to break out the USB microscope and look for clues. Heck, I've got nothing else to do!
But thanks for reading everyone, I appreciate the virtual support!