I've never seen this - what version of Postgres are you using? If it is PG 7.1 and if you can afford to let your system sit in this broken state for a bit and are willing to do a little work to help track down the bug, e-mail Tom Lane at
mailto:tgl@sss.pgh.pa.us and describe the problem and offer your help. He'll be able to figure out just what's causing the problem, and give you some specific advice for working around it.
Don't do this if you're using PG 7.0 or earlier.
If you can't wait and need to fix it ASAP, the first thing to try is to stop the postmaster and restart it. Do this CORRECTLY i.e. via pg_ctl if you're running PG 7.1.
