IMPORTANT EDIT: It is now possible to achieve this with DATETIME fields since MySQL 5.0
Previous versions can't do that with DATETIME...
But you can do it with TIMESTAMP:
mysql> create table testtable (str varchar(32), date TIMESTAMP DEFAULT
CURRENT_TIMESTAMP);
Query OK, 0 rows affected (0.00 sec)
mysql> desc testtable;
+-------+-------------+------+-----+-------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+-------------------+-------+
| str | varchar(32) | YES | | NULL | |
| date | timestamp | NO | | CURRENT_TIMESTAMP | |
+-------+-------------+------+-----+-------------------+-------+
2 rows in set (0.00 sec)
mysql> insert into testtable (str) values ("demo");
Query OK, 1 row affected (0.00 sec)
mysql> select * from test;
+------+---------------------+
| str | date |
+------+---------------------+
| demo | 2008-10-03 22:59:52 |
+------+---------------------+
1 row in set (0.00 sec)
mysql>