I have the following almost working:

sm0 at nubus0 slot d: AsanteFAST 10/100 NB
sm0: SMC91C100, revision 1, 128 KB
sm0: MAC address 00:00:94:77:e8:08, default media MII
nsphy0 at sm0 phy 1: DP83840 10/100 media interface, rev. 0
nsphy0: 10baseT, 100baseTX, auto

It probes fine (as you can see) and I can configure the interface,
but there are still some problems.  I'll be working on it some more,
but wanted to report some success.

I am using the MI driver (with modifications to handle the MII/PHY
interface) and a revamped bus_space implementation that might be
a little slower, but might allow more PC-like devices to work more
easily.  Anyone have a nubus soundblaster they want to ship me?  :-)

-allen