I had the same thing happen to me yesterday after I compiled AOLserver 3.2. I was lazy so I thought of using the nspostgres.so.
It seems that nspostgres.so can't recover from errors... In my case I was modifying the Survey module and forgot to change a constraint, that caused my inserts within a transaction to fail.
Apparently nspostgres.so can't recover from that and the whole AOLserver process goes zombie. Funny is that no error messages (besides "Error:" would be output). The OpenACS postgres.so forwards PG's error message to the logs.