I think that I'm doing something wrong, in hindsight - I just looked at phpInfo, and sure enough:
Loaded Modules mod_fastcgi, mod_jk, httpd_defines, httpdmon, mod_perl, mod_php4, mod_frontpage, mod_ssl, mod_setenvif, mod_so, mod_usertrack, mod_headers, mod_expires, mod_digest, mod_auth_db, mod_auth_anon, mod_auth, mod_access, mod_rewrite, mod_alias, mod_proxy, mod_userdir, mod_actions, mod_imap, mod_asis, mod_cgi, mod_dir, mod_autoindex, mod_include, mod_info, mod_status, mod_negotiation, mod_mime, mod_log_referer, mod_log_agent, mod_log_config, mod_env, http_core
We have rewrite! And just about every other Apache mod written

I think it might be a permissions issue, so I'm going to check that everything is correctly chmodded.
I guess at the end of the day, the easiest way to get in Google's 'good books' is to leave the forum at it's defaults and archive it for indexing instead. That way you can strip signatures, etc out and it's much 'cleaner' - I've noticed lots of other sites do it. I think that doing it constantly would constitute a lot of server load though.