It occurred to me that Perry E. Metzger wrote in gmane.os.netbsd.ports.i386:
>
> Jukka Marin <jmarin@embedtronics.fi> writes:
>> An update:
> [...]
>> The system does _not_ freeze, but the network interface does.
>
> This sounds like A PR I've filed, #34023.
>
> http://www.netbsd.org/cgi-bin/query-pr-single.pl?number=34023

Hmm. On my system I do get those stalls as well, but they're infrequent (Once
every 2 or 3 weeks, maybe), I do _not_ get the watchdog message, and I've
found several peculiarities: 

* Restarting dhclient fixes the problem - at least temporarily.

* the network card isn't *really* dead. When I run tcpdump on it, I still see 
  packets arrive with my IP as destination - it just seems like the NIC 
  doesn't realise it is the intended target. I don't see any packets
  *leaving* the interface using the assigned IP. It makes me wonder if 
  packets don't get dropped somewhere, and it makes me wonder if it's really
  the nfe0 driver at fault.

* In fact, the NIC still works happily on an second IP. My modem uses a 
  private IP for configuration purposes (172.19.3.1), and this seems to work
  just fine if I assign an appropriate alias (ifconfig nfe0 alias 172.19.3.2)
  while the primary (public) IP remains dead. 

I have yet to figure out exactly what dhclient's doing that makes any 
difference. Taking the interface down- and up doesn't help, re-applying the
IP address doesn't help, putting it into promiscous mode doesn't help, so
I'm slowly starting to wonder what kind of magic dhclient is doing :)

Next time it's doing this, I intend to eavesdrop using a third NIC; I hope
I won't be changing the situation too much, though; I'd have to run things
through a hub, and the only _hub_ I have is 10 mbit.

All of this is on an Athlon64, Asus A8N-E (nForce 4, nfe0) running 
current. (using the amd64 port, however).