Hombre, pero si actualizas el registro indicando un valor timestamp diferente al tiempo actual también lo inserta.
Cita: The TIMESTAMP column type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current date and time. If you have multiple TIMESTAMP columns, only the first one is updated automatically.
Automatic updating of the first TIMESTAMP column occurs under any of the following conditions:
The column is not specified explicitly in an INSERT or LOAD DATA INFILE statement.
The column is not specified explicitly in an UPDATE statement and some other column changes value. (Note that an UPDATE that sets a column to the value it already has will not cause the TIMESTAMP column to be updated, because if you set a column to its current value, MySQL ignores the update for efficiency.)
You explicitly set the TIMESTAMP column to NULL.
TIMESTAMP columns other than the first may also be set to the current date and time. Just set the column to NULL or to NOW().
You can set any TIMESTAMP column to a value different from the current date and time by setting it explicitly to the desired value. This is true even for the first TIMESTAMP column. You can use this property if, for example, you want a TIMESTAMP to be set to the current date and time when you create a row, but not to be changed whenever the row is updated later
Más información en: <a href='ir.asp?http://www.mysql.com/doc/' target='_blank'>http://www.mysql.com/doc/...</a>