On MariaDB 5.5.25

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 760505
Server version: 5.5.25-MariaDB MariaDB Server

Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.
This software comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to modify and redistribute it under the GPL v2 license

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mdp01r:(none)> select now();
+---------------------+
| now()               |
+---------------------+
| 2012-10-16 22:46:17 |
+---------------------+
1 row in set (0.00 sec)

mdp01r:(none)> select unix_timestamp();
+------------------+
| unix_timestamp() |
+------------------+
|       1350427587 |
+------------------+
1 row in set (0.00 sec)

mdp01r:(none)> select unix_timestamp(now());
+-----------------------+
| unix_timestamp(now()) |
+-----------------------+
|            1350427590 |
+-----------------------+
1 row in set (0.00 sec)

mdp01r:(none)> select unix_timestamp('2012-10-16 22:46:17');
+---------------------------------------+
| unix_timestamp('2012-10-16 22:46:17') |
+---------------------------------------+
|                     1350427577.000000 |
+---------------------------------------+
1 row in set (0.00 sec)

mdp01r:(none)>

==============

On MySQL 5.5.9

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 7
Server version: 5.5.9 Source distribution

Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.
This software comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to modify and redistribute it under the GPL v2 license

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

root@localhost/(none)> select now();
+---------------------+
| now()               |
+---------------------+
| 2012-10-16 15:54:16 |
+---------------------+
1 row in set (0.00 sec)

root@localhost/(none)> select unix_timestamp();
+------------------+
| unix_timestamp() |
+------------------+
|       1350428064 |
+------------------+
1 row in set (0.00 sec)

root@localhost/(none)> select unix_timestamp(now());
+-----------------------+
| unix_timestamp(now()) |
+-----------------------+
|            1350428084 |
+-----------------------+
1 row in set (0.00 sec)

root@localhost/(none)> select unix_timestamp('2012-10-16 15:54:16');
+---------------------------------------+
| unix_timestamp('2012-10-16 15:54:16') |
+---------------------------------------+
|                            1350428056 |
+---------------------------------------+
1 row in set (0.00 sec)

root@localhost/(none)>