Devil's advocate: but language evolves. (Going to eat some pseudo-academic crow now...) Thank the lord for Daniel Webster - BUT appreciate the ability of language to adapt.
The computer world is full of (sometimes goofy) analogies... desktops, trashcans, log books, sign in, and sign out. We use analogies to help users wrap their heads around CRUD, security policies, http, etc. basically. So I guess if your site is friendly and you want your users to feel like they are signing a guestbook then go for the 'Sign In'. I think that most users don’t really even pay attention to that stuff and they are busy trying to remember what user name / login name / passkey / passcode / password they need to use for your site.
Don't get me wrong, I think that analogies are really important... but I think it’s more important that your site's language is consistent. It can read like the IRS or a bed and breakfast but just keep it consistent.