FAQ for
Ocean's Twelve (2004) More at IMDbPro »

It's called "The à la menthe (the lazer dance version)" and is performed by Nikkfurie of the band La Caution. Both the six-minute instrumental version and the original version, featuring rap by Nikkfurie and his brother Hi-Tekk, are available on their album Peines de Maures (a double CD also called Arc en Ciel Pour Daltoniens).

You can download a two-minute sample of the song for free at http://www.kourtrajme.com/thealamenthe.zip

Although the film does give clues that Ocean and his crew are up to something (i.e. the subtitles referring to the days of the week), it is not really specific as to when they stopped stealing and started pretending. If you follow the storyline the most likely place this occurred was after they tried to rob van der Woude´s house and discovered that The Night Fox had already looted it. The movie however does not show this and cuts to Isabel Lahiri hearing news of the crime. Presumably Ocean must have realized that The Night Fox was always going to be one step ahead and decided to visit Matsui again, who then tells him of LeMarc's plan (this means the police only hear about the robbery a couple of days after it happened, which would be consistent with the fact that van der Woude is agoraphobic and would be very hesitant about contacting people and letting them in his house).

If you read the screenplay, available from Movie Scripts: Drew's Meat and Potatoes http://www.script-o-rama.com/table.shtml, you can see that, towards the end, it was originally supposed to be revealed that Danny and Rusty had been planning it from before they even went to Europe. You will notice, in the flashback that shows them arguing about whether or not to go, that both of them are smiling, because they already have their connection to LeMarc. They just don't want the whole team to know, as it wouldn't be believable to the Night Fox.

Also, it is worth considering that toward the end of the movie, we learn that it is after they try to rob Van Der Woude's home when LeMarc tells Danny and Rusty everything there is to know about The Night Fox, and warns them that from that point on, he would always be monitoring their activities, since the "competition" had begun. This happens right after they get out of the hotel in Amsterdam in the Arsenal's uniforms. In the train station they head off to Paris first, to meet LeMarc, and then to Italy, to meet The Night Fox (and steal his paintings).