Help with duration of statement: EXECUTE [PREPARE: COMMIT]

We currently have

   log_min_duration_statement = 5000

and are seeing statements like the following logged

2010-10-16 05:55:52 EDT [6334]: [1-1] LOG:  duration: 5572.517 ms
statement: EXECUTE <unnamed>  [PREPARE:  COMMIT]
2010-10-16 06:06:24 EDT [26856]: [1-1] LOG:  duration: 5617.866 ms
statement: EXECUTE <unnamed>  [PREPARE:  COMMIT]
2010-10-16 06:06:24 EDT [20740]: [13-1] LOG:  duration: 5210.190 ms
statement: EXECUTE <unnamed>  [PREPARE:  COMMIT]
2010-10-16 08:24:06 EDT [8743]: [1-1] LOG:  duration: 6487.346 ms
statement: EXECUTE <unnamed>  [PREPARE:  COMMIT]

Questions;

1) What do these statements mean?
2) How do I dig deeper to determine why they are taking longer than 5 secs.

Version Info -->

select version();
                                                   version

------------------------------------------------------------------------------------------------------------
  PostgreSQL 8.1.17 on x86_64-unknown-linux-gnu, compiled by GCC gcc
(GCC) 4.1.1 20070105 (Red Hat 4.1.1-52)