The API is already available although we haven't announced it. There is a WordPress plugin that uses it, called TagMahal.

Please contact us if you'd like to use the API, if you need to do up to 5k queries per day or so it shouldn't have much of an impact on our server.

I would like to see a Blogspot plug-in too! I just tried it on my latest post and it worked much better than I did. :)