Dump the plain text:
pg_dump mydb > db.sql psql -d newdb -f db.sql
This should work fine as it does not store version information and uses a simple SQL format in the dump. Data is being restored with COPY so quickly. If you used some functions of 9.0 in the database, you will have to manually edit the scheme in the dump so that it works on 8.3. However, Blobs cannot survive in this format; I have not tested this).
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