8.1: import of 8.0 dump fails with UTF-8 error

Hello,

I did a pg_dumpall of all my Pg 8.0.3 databases, removed 8.0, installed
8.1 and tried to import the dump. One table in one database failed with:

ERROR: invalid UTF-8 byte sequence detected near byte 0x83
CONTEXT: COPY pwd_name, line 22428, column name: "t.tonnement"

So I exported that database with 8.0 as Inserts to a text file and tried
to fix it using iconv, but that fails as well:

# iconv -f UTF-8 -t UTF-8 dump.sql > dump-fixed.sql
iconv: illegal input sequence at position 2588882

How can I fix the sql script to import it?
I have Debian Linux 3.1.

Thanks a lot,
Thomas