As TechCrunch reported: Pigs Fly as Facebook and Google work together on an Android App - there's been a few indicators that this might be happening, particularly with some random mentions here and there on Twitter but no one was really expecting anything to occur given the competitiveness between the two businesses.
What's most interesting about this (particularly from my standpoint as an Android user) is that it will be the only other official mobile client besides iPhone; which really endorses the Android platform as the second runner to iPhone. And in acknowledging that, it also indicates that Facebook are considering that Android will have substantial traction in the coming year - not least when you consider there are two dozen Android based phones slated to hit the market in the rest of the year which could make a serious dent in iPhone's penetration.
iPhone launching with the Facebook client has been largely cited as one of the big levers in it's sales. Smart phones have been around for a decade and there have been sleek devices previously (Nokia 7710 for example was just one big touch screen 3 years before the iPhone launched) but the mind of the consumer wasn't fired by the opportunities it could provide to them. You only had to look at the marketing by the telco's around the iPhone launch to see that the Facebook client was the Killer App for the smart phone in terms of hooking people in. It gave them a very tangible benefit to owning what would have been the most expensive handset they'd have bought to date - "I can keep in touch with my friends besides calling them..."
With an official Facebook client for Android, the same endorsement has been conferred and one of the key marketing differentiators has been removed. I'm tipping late 2010 to be an interesting time as Apple and Google really go toe to toe and start slugging it out - which will be fantastic for innovation in this space.
Source for citation: ajfisher.me/2009/07/29/an-unofficial-endorsement-of-the-android-platform.