Just as an update for anyone else who ever does anything with a PS/2 port, there are machines out there which have non-standard keyboard controllers.
The main differences I've found so far:
- the clock line is driven from host (not the keyboard)
- the clock line runs constantly (even when the keyboard is idle)
- the data line does not necessarily return to high when idle (most important when trying to detect that the host has entered 'request-to-send' state)
The annoying thing is that the keyboards attached to these controllers still work fine, so you have no way of knowing whether you have one or not until you hook a scope up to it.