When does postgres set the connection state back to idle?

I've been having a difficult time finding an answer for this via google, so please excuse me if this is the wrong list, or a stupid question...

We're trying to diagnose a system issue where we ended up with 100's of non-idle queries in pg_stat_activity, but the actual system load didn't correspond to what we should have seen with that query load. Is it possible that a client could cause this by not acknowledging that it has received results, so that postgres will think the query is active even when it's done the hard work of getting the queried data off disk? These were all SELECT queries if that makes any difference.

-Stephen