<p>Is it possible to define a timestamp column in a MySQL table that will automatically be updated every time a field in the same row is modified? Ideally this column should initially be set to the time a row was inserted.</p>

<p>Cheers,
Don</p>

That is the default functionality of the
timestamp
column type.  However, note that the format of this type is yyyymmddhhmmss (all digits, no colons or other separation).
EDIT:
The above comment about the format is only true for versions of MySQL < 4.1... Later versions format it like a
DateTime