I/O error on data file, can't run backup

Running postgresql 9.0.5 on

balapapa ~ # uname -a
Linux balapapa 2.6.39-gentoo-r3 #1 SMP Sun Jul 17 11:22:15 CEST 2011 x86_64
Intel(R) Core(TM) i7 CPU 930 @ 2.80GHz GenuineIntel GNU/Linux

I'm trying to run pg_dump on my database, and get an error:

pg_dump: SQL command failed
pg_dump: Error message from server: ERROR:  could not read block 1 in file
"base/612249/11658": Inn/ut-feil
pg_dump: The command was: SELECT tableoid, oid, opfname, opfnamespace, (SELECT
rolname FROM pg_catalog.pg_roles WHERE oid = opfowner) AS rolname FROM
pg_opfamily

I have tried to stop postgresql and take a filesystem backup of the data
directory with a cp -ax, but it crashes on the same file. I've looked at the
directory with ls -l, and the file looks pretty normal to me. I've also
rebooted from a live CD and run fsck on my /var partition, and it doesn't find
any problem.

The database is still working perfectly.

The backup script overwrote my previous backup with a 40 byte file (yes silly
me I know that's bloody stupid - I'm gonna fix that) and now I haven't got a
recent backup anymore.

Is this fixable?

regards, Leif