replay doesn't catch up with receive on standby

This is on 9.0.3: I've got two dbs running as standby to a main db. They start up fine and seem to think they're all caught up (by /var/log logs), but

SELECT pg_last_xlog_receive_location() AS receive, pg_last_xlog_replay_location() AS replay;

reports replay behind receive and it doesn't change. This is on both dbs.

Notably the main db isn't (wasn't) doing anything, so no new commits were causing things to move forward. I did a write to it and both slaves moved both their recieved and replay serial numbers up.

Is there a valid situation where an idle master/standby setup could remain with replay behind received indefinitely? (My nagios monitor isn't very happy with that (at present)) and before changing that I'd like to understand better what's going on.)