Auto increment field when updating?

Hi, hope you can help. This may be a really stupid question, but here
goes....

I have a table called 'log'. This keeps track of an ongoing process.
The process writes records to the 'log' table using a unique id. If the
id does not exist then the process inserts, else it updates the existing
row. Only some fields are written to each time the row is updated.
There is one field called 'count' that ONLY needs to increment every
time that particular row AND the field 'count' are updated.

I know that I can select the information, increment it and update it
back again, but is there any way of setting up the database to do this
automatically? I am using PostgreSQL 6.5.

Thanks