As far as forum rules go, I think as long as no-one posts direct (or indirect) links to BIOS or ROM files, the discussion is fine. Other staff are free to overrule me on this.
On the matter of legality, there are 2 parts to it - hardware and software.
Hardware is the physical component that you buy and can touch. To the best of my knowledge, there is nothing illegal about making a software emulator for a random piece of hardware.
But, when you buy a console, the software (henceforth called BIOS) which runs on the hardware is proprietary material. It's owned by the console manufacturer and they probably have copyright protections over it. Simply put, unless Sony/Nintendo/Microsoft gives you written permission, you are NOT allowed to make copies for mass distribution.
This is why most emulator developers insist that you get a 'bios' file from somewhere else. They can legally develop the emulator, but its illegal for them to do anything with the BIOS that runs on the hardware.
The issue of region-locking makes the question of BIOS ownership tricky. For example, if you own a PS2, then there should be no legal harm in getting a copy of the BIOS, right? Well, what about a situation where you are in Europe with a PAL-version of the PS2. Would it be illegal for you to get a copy of the NTSC-version of the BIOS? I mean, you do own the console, so what's the harm?
Questions which I really don't know (or want) to answer.
-chronodekar