Ask HN: User authentication via Facebook and Twitter?

Hi all,

I've been working out an idea over the past few days. It would essentially be a social network of sorts, but I am not trying to create a new one per se. I'd like to leverage an existing social networking sites user authentication. I'm thinking of allowing Facebook or Twitter logins.

Are there any gotchas to doing this? Anything that I should be aware of that might not work so well if I were to use these user authentication APIs from Facebook or Twitter?