Set flag column on update if other columns change?

I've got a table where updates that actually change things need to
record in the row that the row has been modified.  I'm assuming that a
trigger function (OK to be plpgsql, performance isn't much of
an issue) is the best way to do this, but haven't done much
with trigger functions.  Can someone give me some pointers
on how this might be accomplished?  I have to check for
changes in a subset of the columns (e.g. ignore timestamps, etc.).

Thanks in advance!
Steve
-- 
Steve Wampler -- swampler@noao.edu
The gods that smiled on your birth are now laughing out loud.