I've been using my alpha with 1.6-STABLE, for pppoe internet sharing for a 
couple of months now.  The box has been working reasonably well for this 
purpose, but I seem to get a lot of these kernel messages:

sip0: transmit FIFO underrun
vr0: unable to allocate Tx cluster
WARNING: mclpool limit reached; increase NMBCLUSTERS

(not necessarily in that order, but there are quite a few of each of these)

sip0 is a neatgear card, vr0 is a D-Link 530TX

Also, not sure if it's related but, I've experienced some very flakey NFS data 
corruption problems when connecting from other machines.  For example, when 
sharing my pkgsrc with my dreamcast, the fetched packages (when fetched from 
the dreamcast) will be corrupted.  I can fetch them from the alpha, and they 
come out fine.  After they are fetched, if I try to do a "make install" from 
the dreamcast,  they'll corrupt during extraction.

If it helps, my kernel source is from Feb 21st, uname says it's 1.6.1_RC2

- Pat