The terminal Ms are AT-compatible, just a different plug and a couple software-interpretation bugs.
Otherwise known as compatible.
So, with a cable swap it will work in your bios and likely bootloader and even DOS - on a PC.
However, the PS/2 drivers are bugged against terminal boards in both windows (all since '95, I believe) and linux, though we've nearly managed to get a patch into the stream which will support it next linux kernel cycle.
Driver patches are being maintained by for both linux and 'dose, at present, though not for hackintoshes. As fer BSD, no clue.
There are also two teensy (and++) implementations, for converting directly to USB, as well as (I think) a hoary kbdbabel converter to normal PS/2.
Both teensy implementations are not-quite released as open-source yet, but can be had from the folks on the IRC channel #geekhack if you can stand it there.
the kbdbabel implementation (if it exists) is hampered by the current CVS security problem on SourceForge, and the fact that it would use an 8051 which is often a one-shot, burn-wise. You want to buy a few, since if you flash them wrong then they are permanently flashed wrong. They are also tiny and old, and only programmable in assembly, so you won't have use for them for other projects if you get bored of the keyboard conversion.
So, compatible, sort of... different amounts of work to get going, depending on how much fun you want to have. That said, a number of us use the terminal IBM boards (F and M) as our main keyboards after the conversion - they are truly awesome, with extra special sound and feel.
Be sure to ask us more if you decide to reacquire the beast as a conversion project. We are always delighted to help corrupt new folks.