In an interview with Total Film while promoting Hercules, Dwayne Johnson was asked about these rumors. He doesn’t really dodge the question at all, but acknowledges that he and DC have been talking for a long time and that they will be announcing his character very soon. He says it isn’t Green Lantern, but it’s a character with the power of Superman.
The only DC character that really fits, which is rumored for a movie, is Shazam. And casting The Rock is a great choice for Shazam. He can really pull off the idea of a kid being in the body of a super-powered adult, and this just makes the Shazam movie even more exciting to see.
Here’s the interview: