Well, I had a site running vBulletin for some time (6 years) and moved it to a new domain opening the site up for more people (It was for a large town, then moved to discussion about the county). I then forwarded my old domain to the new one. I then thought i'd start again with my town site using XenForo (Keeping the vB one for the county) and already posts and threads relating to the town are showing up in Google. It doesn't even have a sitemap! Got to say, I love this software. It's nice, easy and simple to use and it seems search engines, if not just Google, like it also.