I'm in the process of converting over my 8-speed Sora/Tiagra-based Specialized Sequoia, to 105 components. Everything I've ordered is the 9-speed variety (as opposed to the 10). I was going to keep my Sora Shifters and rear cassette for now.
I don't know if I need to use an 8-speed chain (to match rear cassette) or a 9-speed chain (to match front chain rings and rest of components). This is the last stumbling block and my project will be complete... well, ordered anyway.