I can confirm this bug.  I have the same problem, and assisted a friend in
fixing it on his machine. (mine is still toast)

My setup:

com 0,1 : normal settings.
com 2: irq5
sound blaster: isapnp jams it on 5 and blows away the com2 (my modem) com2
becomes useless, and the sound blaster works great.

My freind's setup:

com0,1 : normal settings.
com2: BEST 33.6 isapnp modem.  Gets totally confused and configures itself as
com1 and fails to attach.  Wedges system on ttyflags.


Looking over the code in isapnp (trying to figure out how to fix his modem) I
found that modems and com ports are configured to be EDGE.  (which is correct)
and that the isapnp driver seems to look for edge devices to share interrupts. 
Unfortunately, modems and com ports are not happy busmates, and will clobber,
or be clobbered by whatever sits with them.  If I turn on my soundblaster pnp,
in spite of all the lovely free addresses I have for it to configure at, It
still likes to pick irq5, and sit on it, and clobber the modem.

I fixed my friend's machine by disabling com1 from his motherboard.  The best
modem then took the slot it wanted. (it *wants* com1 and it takes it by force,
it will fail the attach, but configure on com1 wether it works or not and hose
everything)

I fiddled a little with it, but couldn't come up with anything.  I'm assuming
it has to do with edge sharing, and the cards preferences about where they sit.
Tracing through the code it *should* have worked, but it obviously lets the
card tell it what to do for some reason.

---
Tim Rightnour    -  root@garbled.net
http://www.zynetwc.com/~garbled/garbled.html