There is one PCI card that I know of that comes with a NetBSD driver:

  http://www.sangoma.com/main/products/cards/adsl

The driver is i386 only so far (and they don't mention NetBSD on their home
page). But engineering there was pretty responsive, I bet if you ask
they'll create binaries for other archs as well.

While testing, unfortunatley I couldn't get it to work with a german standard
ISDN/DSL line (with bandpass splitter) though - and I have no access to a
"simple" DSL lines. Maybe their stuff is incompatible with the signaling in
use by t-com (I don't know anything about the atm side of things).

Martin