> #5  0xc0102e39 in calltrap ()
> #6  0xc02b8cdc in pppoe_start (ifp=0xc0a24800)
>     at ../../../../net/if_pppoe.c:1413

This means it crashes in the first call to pppoe_output(). Could you try
adding a few printfs to that function and see how far it gets there?
If you have another NetBSD machine and a serial connection, you could set
up kgdb and manually step through it (contact me off list if you need
instructions).

I don't have any USB ethernet devices, so I can't reproduce it myself.

Martin