Ted Lemon writes:
> Irix does this as well.   It's been requested as a feature for NetBSD
> here at vix.com.   I don't know of a way to do it now, but I'd love to
> hear of one!   :')

Use ipfilter.

I have the following in /etc/ipf.conf:
	count in on ppp0 all
	count out on ppp0 all

And when I do ipfstat -aio (`a'ccounting info, for `i'n and `o'ut), I get:
	35479553 count out on ppp0 from any to any
	30933385 count in on ppp0 from any to any

(that's in 7 days over a 28.8k PPP link)