It occurred to me that David Brownlee wrote in gmane.os.netbsd.general:

>  	You may want to reduce the read & write block sizes down to 1k
>  	or so -  (mount -o -r=1024,-w=1024). See mount_nfs(8) for more
>  	details.

The plot thickens. When I do that, I get a kernel panic - but only when 
using the wireless device. And it's not just NFS; samba does the same 
thing. Again, only using atu0, not using fxp0.

I have a nagging suspicion that there's slightly more afoot. For the record,
my laptop is an IBM Thinkpad A31. I'll try reducing optimisation flags, and
see what that brings.