Stackoverflow does exactly that. They have big buttons for all the major identity providers, including Google, Facebook, etc. and a text field at the bottom for manual openid login.

Personally I feel a lot better about Twitter + OAuth than FB, but that's just me.

As for why not use OpenID? Personally, I don't want to tie my Gmail account to a lot of things -- then people have my email! I don't necessarily want to tie my Facebook to them, either, but in the same breath some forms of OpenID can end up sharing more, depending...

This is why I get cranky when people implement Disqus, too -- old comments I made that were supposed to be anonymous but which had my email for the author, etc, are now pulled in and linked to a general identity months or years later. :(

Why would you feel better about Twitter + OAuth than FaceBook + OAuth (which is what the sign on component of FB Connect is)? From a technology perspective, it's the same protocol. Neither company has a good security track record.

(not criticizing BTW, just curious)

the twitter account could be a throwaway account and generally lesser risk of anybody getting too many personal details.. facebook, not so much ..

Because all the major sites/companies were too busy fighting to be the provider everyone uses and never got around to actually accepting anyone else.

But that isn't really an issue if you want to accept open ids on your site - it is actually a good thing, because the more providers there are the easier it is to find on that has your customers.