Slow COMMIT statements

Hello List,

2011-12-07 11:52:52 CET 8075 xxx [local]LOG: duration: 1168.953 ms
statement: COMMIT
2011-12-07 11:52:52 CET 7327 xxx [local]LOG: duration: 1116.119 ms
statement: COMMIT
2011-12-07 11:52:52 CET 31727 xxx [local]LOG: duration: 1131.044 ms
statement: COMMIT
2011-12-07 11:52:52 CET 25077 xxx [local]LOG: duration: 1064.599 ms
statement: COMMIT

these messages show up in the postgresql logs at irregular intervals. We
got no slow queries before or after these statements. Checkpoint logging
is enabled also but these don't show up either, well they do but minutes
later ;).
I would like to debug these slow COMMITs but don't know what is causing
them.
The server itself is idling most of the time... with a load avg around
1-2(with 64 cores) and iowait around 0.5 - 1%.

Notable changes from default config:

log_min_duration_statement = 500
log_checkpoints = on
checkpoint_segments = 40

Regards,

Felix