I've bricked my phone in a peculiar way - it appears to be a hard brick, non-responsive to any buttons. Plugging in to the computer gets me the dreaded qhusb_dload QPST download mode. Not a good sign.
It wasn't due to faulty ROM flashing, either. I made a bad edit to /system/etc/vold.fstab fiddling with the way the internal and external SD cards get mounted - mmc1 instead of mmc2. After one reboot, I had some sd card corrupted errors. Proceeding to format the card may have been my fatal error. By the second reboot, I was bricked. D'oh. I imagine I had accidentally mounted a partition of the internal emmc that is critical to normal operations, then tried to wipe it.
From what I've read, a USB jig won't do much good, but it's worth a try.
Anyone messing around with their /system folder, be warned.