Google has a mailing list you can sign-up for to receive more information in the future.Google Plus doesn't have a public API yet, nor has it announced when one will be available. But if you want to find out about future developer opportunities,
Developers are already clamoring for an API, which is a good sign for Google Plus. Mohamed Mansour commented "Would be nice if there was a Google+ API. I would have made a Buzz to Google+ importer today."
What can we expect from an API once it becomes available? Google's Joseph Smarr told ReadWriteWeb's Marshall Kirkpatrick "we're eager to make the social graph a 'two-way street' where you can use your circles to quickly get up-and-running on a new site, but also make new friends on that site and add them to your circles. Lots of details to work through, but the best way to do it is with good agile partners building cool social experiences." Marshall compared it to Facebook Connect.
We hope Google Plus borrows a page from Google Buzz and employs open Web standards such as Atom, Activity Streams, MediaRSS and PubSubHubbub (see our previous coverage of standards support in Buzz).