A few months ago a source revealed to this blog that Linkin Park was once again tapped by Michael Bay to provide a song for the Transformers: Dark of the Moon soundtrack. At the time, it was said the single would be Burning in the Skies from their A Thousand Suns album. That was never officially confirmed but band member Chester Bennington did hint that a third collaboration was a strong possibility.
An update to the official Linkin Park website confirms the they are providing a single for the movie with a moon icon that when clicked brings you to a larger moon image that turns into a multi-colored Autobot logo while a few chords of a song plays. The song has been identified as Iridescent, also from A Thousand Suns. It could be a feint but probably not a reach to assume that the song is the chosen single for Dark of the Moon.
In addition LP's Joe Hahn has tweeted that they might be working on the music video for the movie when he posted "weather is good. Might be a good week to shoot a music video" followed a day later by "Do robots and leather look good together?" Amazon has a sample from the song. (via TFW2005, thanks to Sahara for link)
Update: A source has indicated that Iridescent will be remixed for the movie, mostly to make it more attractive for radio. In general most radio stations play a certain "type" of music (rap, alternative, top 40, adult contempoary, oldies, etc) and these types can often overlap. It is in the best interest of sales to make a song overlap as many types as possible to maximize the number of stations that will play it helping in it gaining traction and popularity which in turn feeds album sales. It also has the not incidental side effect of helping to promote the movie. Sabi in the comments posted a link to site claiming to have that remixed version. I don't own A Thousand Suns album so don't have way to compare songs to hazard a guess on this being true.