There has been an immense amount of speculation over the integration of Facebook into iOS 6 recently, especially after Apple CEO Tim Cook’s teasing of a partnership at the All Things D conference this week. Now, MG Siegler reports that Apple will indeed be adding Facebook integration in the newest version of its mobile OS.

Siegler says that the integration deal is all but done, but also notes that these things can always change with Apple:

Now the two sides appear on the brink of formalizing the relationship. After much speculation, Facebook integration will indeed be baked into the latest version of iOS, we’ve learned.

It’s important to note that Apple being Apple, something could change in the next week and a half (see again: Facebook/Ping). But as of right now, Facebook is a go in iOS “Sundance”.

We conjectured about Facebook being one of the things Apple would fill all of those TBA sessions at WWDC with yesterday.

Apple and Facebook have clearly been working more closely together in recent months, as Facebook’s Camera app was given top billing and a name that mimics Apple’s stock app. This kind of cooperation doesn’t happen without some sort of positive relationship.

Facebook’s framework, as Siegler notes, is significantly more complicated than Twitter, Apple’s first social partner. This should make it interesting to see how Apple plans on integrating the sign-on and sharing features of Facebook into iOS and might pose a challenge for developers.

The report goes on to say that sharing in Mountain Lion will remain Twitter-only for now.

Since the moment that Twitter scored its immortality clause with integration in iOS 5, speculation has been rampant of Facebook following suit . Evidence of its integration has been found in pre-release iOS software, but it was nowhere to be found when either iOS 4 or iOs 5 launched.

Twice recently, once at a shareholder’s meeting in February and once at the All Things D appearance, Cook has said that he has always “thought the two companies could do more together,” adding “stay tuned” to his statement most recently.

I’ve said it before, but I think that whatever issues there were between Apple and Facebook over the use of the API for Ping sharing feels like ancient history. The time seems right for this to go down.

Twitter integration with iOS has benefited the service in a big way, with signups up 25% worldwide after integration. Facebook has a lot more users that Twitter, but its mobile offerings have left a lot to be desired on iOS. This could enable iDevice users  to shove data-filled images and status updates out to Facebook in a fluid way that the company’s official apps haven’t been able to deliver.