mod_rewrite

I use it very frequently. It helps some search engines and has a negligible difference on others. For search engines that don't like URL variables you can work around them.

Google does fine with most URL variables.

I don't recommend using it for phpbb unless you know what you are doing. By stripping sessions the search engines can already index your pages so it would really have very small benefits (e.g. some people contend that search engines prefer the .html extrension).

someone wrote a mod or something at the phpbb forum using mod_rewrite, I should look into it. Unless you already have.

Someone posted some mod_rewrite code on the phpbb SEO thread. I haven't looked into it much but it looks like better code than the other mods I've seen.