An extension of the "Is it only the postgres user..." question above.
I've found that the log file produced by postgres gets really huge. I changed my startup file to divert the postmaster output to a file rather than /dev/null. It gets huge when you've had postgres up for a while. So the first thing that comes to mind is to check if the output from postgress is being sent to a file. And if it's over the quota or just generally huge.
On a side note/question. Does anybody have a good strategy for rolling the postgres log? Is there some better way to do logging with Postgres?