so, after i discovered that my friend's 1.3.3/sparc box was ok,
i decided to test a few things myself. here's a summary

	NetBSD-1.3.3/sparc  with le0	good
	NetBSD-1.3.3/i386   with we0	bad
	NetBSD-current/i386 with we0	bad
	NetBSD-current/i386 with ex0	good

we0 is either a SMC 8216 or 8013; I've tried both.
ex0 is a 3com 3c905.

So, it looks like the problem is the we driver. However, it's only bad
when connected to the cable ISP's hosts. To recap:

	netbsdbox <-> cable modem <-> cable isp <-> internet <-> other_hosts
					 |
				    cable_isp_hosts

* ftp between netbsdbox/we0 <-> other_hosts is ok; 200k/s, as I'd expect,
  given that I think the link from the ISP to the internet is a 2Mb link
* ftp between netbsdbox/we0 <-> cable_isp_hosts is bad; 1.5k/s
* ftp between netbsdbox/ex0 <-> other_hosts is unknown (i didn't test :/ ),
  but i'd expect it to be 200k/s
* ftp between netbsdbox/ex0 <-> cable_isp_hosts is good; 600k/s

Note that the cable ISP blocks ICMP to their hosts (e.g, ping, etc),
which I can't imagine causing a driver specific problem (but you never know).

So, it looks like a bug in the `we' driver here. Whilst I can put a 3x905
card in my box, I think that shipping 1.4 with a dodgy we driver isn't
something we should do, unless we put something in the release notes like
``this card is supported unless being used to connect to Telstra's Big
Pond Cable'' ;-|

Any thoughts on how to debug this?