I think I'm one of the few big sites around still using .shtml, so I was just wondering if there is any kind of SEO impact from doing that? I don't see too many sites use it these days...
It should pose no SEO problems at all. Server Side Includes are not different from php includes or any other form of templating from a user stand point. I think part of the reason why ssi's go unused is due to their limitations. They're great at including a navbar or some other bit of html that you want to include across the entire site, but if you so many pages that manually editing is an onerous task, you could just as easily install a CMS. But the virtual and exec directives pose such a great security risk that webhosts will often disable ssi entirely (because it's much easier to disable entirely than selectively disable certain commands).
From Matt Cutts (I watched one of his videos recently): File extensions have absolutely no relevance to how a page ranks, unless it is an EXE or something that might be a virus, etc. Just an aside, you do know that you can use server-side includes without the shtml extension right? As long as your web server is configured properly, you can use SSI in regular HTML files. We do it, and it works fine.
Thanks guys! I know we can use shtml without the shtml extension but we have some custom server config's that don't allow for it. We'll be moving to php shortly anyway, but thank you for the help!
I myself still use SSI for many of my sites, just configure your htaccess file to read html files w/SSI.