Okay, so I've gotten this into a state where it works without the Mac.

Or with the Mac.

But not during a transition.  All I have to do to turn the card into a
smouldering heap of melted plastic is go over to my Mac and either "Turn
Airport On" or "Turn Airport Off".

I tried setting WI_INTRS to 0x7FEF, to see if the card had interrupts
it needed handled (other than WI_EV_TICK or WI_EV_CMD, which happen
too often, and which weren't in WI_INTRS before).

Nope!  When another card joins or leaves the network, no interrupt is
registered.  Nothing seems to happen at all - but every attempt to send
the card a command after this fails because the card is busy.

I did notice that the TICK bit is often set when the interrupt comes in,
so I started handling it (just doing WI_EVENT_ACK), but that doesn't seem
to change anything.

Does anyone have any idea what might happen to one of these cards when
another card joins or leaves the area?

-s