Excerpt from http://www.mysql.com/doc/D/A/DATETIME.html:
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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 than 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:
Let MySQL set the column when the row is created. This will initialise it to the current date and time.
When you perform subsequent updates to other columns in the row, set the TIMESTAMP column explicitly to its current value.
On the other hand, you may find it just as easy to use a DATETIME column that you initialise to NOW() when the row is created and leave alone for subsequent updates.
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I hate to say it, but when something is this easy to find why bother asking a question? All I did was go to www.mysql.com and type in timestamp in the search box, and this information was under the first result returned.
So incase you did read this and just don't get it, you can add the field to your update statement in both the columns and values section which will set the timestamp to it's current value and not change it.
