iTunes previews are integrated right into the Twitter interface, thanks to a new partnership between Twitter and Apple.

There's a crucial new partnership for Ping, the social music service that Apple launched as part of its iTunes 10 unveiling this summer: Twitter users can now link up their accounts.

"Once you've linked the accounts, whenever you post, like, review, or tell your friends why you purchased a song or album on Ping, this activity will also be tweeted to your Twitter followers--complete with playable song previews and links to purchase and download music from iTunes," a post on the Twitter blog explained.

This is big, because Ping was one of those rare Apple product launches that legitimately underwhelmed, largely because the long-rumored iTunes social-networking initiative did not sync up to either Facebook or Twitter. Apple said at launch that Ping users would be able to sync up with their Facebook friends, something that never came to fruition. Apple CEO Steve Jobs hinted that negotiations with Facebook had soured, citing "onerous terms" on behalf of the massive social network. It's not like Facebook and Apple have never partnered before: Facebook Connect is available for iPhoto uploads, and there's a separate version of the universal log-in tool just for the iPhone.

But Twitter's terms are apparently more flexible, and they're even integrating an iTunes song preview into the Twitter.com interface, adding yet another feature to the "New Twitter" redesign that the company launched several months ago.