Another option... I don't know if MySQL has the NVL function, which returns the first argument if it's not null, and the second argument otherwise. With Oracle, you could do something like this:

$insert_sth = $dbh->prepare(<<"EndOfSQL");
  INSERT INTO members
    (member_id, name, date_joined)
  VALUES
    (?, ?, NVL(?, SysDate))
EndOfSQL

$insert_sth->execute(1, 'Bob', '2001-08-15');  # Bob actually joined y
+esterday
$insert_sth->execute(2, 'Tom', undef);         # Tom is joining now
[download]

So, you can bind a specific date in the execute, or you can bind undef and let the database specify the current date.