Hello,

> >Digital I/O on the NET4801 was added recently:
> >http://mail-index.netbsd.org/current-users/2005/09/27/0016.html
> >Adding I2C to this should be nearly trivial.
> 
> I know what I2C is, and as I understand it, being a I2C master is
> relatively easy.

Yes, and kernel support is there so all you need to provide is
a handful functions to read/write the IO pins.

> Are there I2C A/D converters?  If they're in chip form, that's okay.

Plenty. The voltage/temperature monitoring chip on your mainboard
probably is one ;) But I guess what you're looking for is something like
this: 
http://www.standardics.philips.com/products/adcs/
or this:
http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?templateId=4&paramTable=yes&familyId=390

These don't have digital outputs but you can always use the Soekris I/O
pins or more I2C stuff.

have fun
Michael