I think you will need to make a decision as to whether you are hoping to serve as a proxy login (you log into me with twitter ; i log into them on your behalf)

I think you will need to make a decision as to whether you are hoping to serve as a proxy login (you log into me with twitter ; i log into them on your behalf) or to have a separate standalone service that's clearly demarcated from the host publisher. I believe you'll find the former is challenging. It would be against the TOS of most authentication providers to do this as an arm's-length intermediary. I think you could do it as the contracted agent of the publisher (which implies a white-label approach). See, for example, the way JanRain works. The best examples of the latter I can think of are Disqus, Echo, et al. You might want to check them out.