I've forked punbb to support multiple sites within one codebase, as well as use two different mysql data sources - one primary and one replicant.
The database has some problems:
I'll fix these, and release the modified codebase and maybe patches too.
UPDATE: September 17, 2007: I think I fixed the double post issue - just needed a "DISTINCT" keyword in the SQL.
UPDATE 2: Punbb admin requires the use of the http_referer request header. Doh! Hopefully I can remove that dependency.