What does SET timestamp=foo do and why is it in my slow query log?

Question

Possible Duplicate:
How can “set timestamp” be a slow query?

My slow query log is filled with entries like this:

# Time: 110506  2:09:14
# User@Host: foo[foo] @  [10.176.163.183]
# Query_time: 2.259825  Lock_time: 0.000000 Rows_sent: 0  Rows_examined: 0
SET timestamp=1304647754;
COMMIT;

What is that command doing, and why does it take so long (sometimes 12 seconds)?