It could be true according to a report at comingsoon.net.

Last week, when Disney announced rather vaguely that Johnny Depp would be playing the role of Tonto in their big screen reinvention of The Lone Ranger, based on the popular '50s television of the same name, it took the entertainment world by surprise.
Maybe people were just shocked that a superstar like Johnny Depp might play a sidekick in any movie he appeared in, not to mention the whole fact that traditionally, Tonto was a Native American who barely ever used prepositions. The real question was who would play Tonto's better half and the title character in the movie that's yet to name a director.
Well, now England's semi-tabloid newspaper The Sun is claiming that Tonto's Kemosabe will be played by none other than George Clooney!
This information didn't come from Clooney himself or from Disney itself, but from an unnamed insider, which makes us skeptical, but here's what they say:

An insider said last night: "George is very keen he's been trying to do a movie with Johnny for a long time. He thinks they'll make the perfect on-screen partnership."

Written by Ted Elliot and Terry Rossio, who also penned the three "Pirates of the Caribbean" movies, the Jerry Bruckheimer production will try to revive the iconic character who first appeared on the radio in the '30s but became best known by the version played by Clayton Moore in the early days of television.
Clooney's last attempt to play an action hero was the ill-fated Batman & Robin for Warner Bros. where he had Chris O'Donnell as his sidekick, so hopefully Disney's movie will fare better for the star.
Look for more news on this exciting new project in the coming weeks, hopefully including some sort of confirmation of this breaking rumor.