I don't know if AutoCAD supports ink or not. That would be the best
option for your client.
Alternatively, your client and print the drawings to OneNote (2003 or
2007) and annotate them in there. However, the problem with that is that
he'll see big drawings scaled down to a normal Letter-size page and
details get lost when zooming.
You can give OneNote a shot, but I am really not sure how well it will
work for CAD drawings.
If you go with ON 2003, you'll need to get the MS Education Pack for
Tablet PC. It includes the "Send to OneNote 2003" printer which lets you
print to ON on a tabletPC.
If you go with ON 2007, a similar printer comes in the package.
If I were you, I'd test this with ON 2007 on a test machine first to see
how the quality and size works for your client. With 2007, you can test
this on a desktop or virtual machine. It'll show you sufficiently enough
whether size & quality are appropriate for annotating.
Patrick Schmid