hdparm -d1 /dev/hdaturns on UDMA for
/dev/hda. (It wasn't on by default for me).
To test to see if it's working, use
hdparm -Tt /dev/hda. If you only get 3 megs/sec when reading, either UDMA is off or you are using a really old drive ;-)
tech notes and web hackery from a new zealander who was vaguely useful on the web back in 2002 (see: python community server, the blogging ecosystem, the new zealand coffee review, the internet topic exchange).