ASFAIK all software bricks are recoverable if you have BootMii installed as boot2 and create a NAND backup with BootMii. Hardware bricks are, of course, unrecoverable without replacement hardware, or understanding the circuitry to fix crossed/short circuits in some cases.
It's nice to know that electronics can magically start crossing leads and such.
In all actuality, something like that is entirely possible but I have never heard of that happening on the wii. I had a laptop once that overheated so bad, and for some reason the hardware didn't just shut down. In the end there were a few surface mount resistors and such that were really close to the problematic IC and the solder melted and things started to slide out of place. Eventually it caused shorts and fried up the laptop beyond repair. It was funny to sit back and watch it happen before my eyes like that. I've heard of that happening with hard drives too, but not yet for a wii.
It's common with people doing their own modchip installs. I don't recommend modchips at all, they're unnecessary these days. Of course, someone is going to walk into this thread and give their argument for modchips, and that's fine.