In my experience the pcmcia stuff is very fragile.  The slightest wrong move
will leave your machine unusable.  You must be very careful to bring the
network down completely before powering the modem down.  You must not let
your machine go into any kind of sleep or suspend mode while the modem is
on.  I've never had any luck swapping pcmcia devices, hot or cold.

Before NetBSD we used Mach, and its pcmcia support was rock-solid.  It had
lots of nice features, like not disconnecting the modem when you reboot
(some people claim this is impossible; I invite them to my office for a
demo).  I could power down the entire machine except for the modem, for days
at a time, and when I powered back up everything worked.

Mach took a completely different approach.  Instead of treating the pcmcia
bus as a bus, it provided an interface to it from user land.  That's it.  A
user program talked to the card and mapped it into isa bus space.  From
there the regular com driver took care of everything.  That may not fit the
NetBSD architecture, but it certainly worked much better.