Electronic Arts attempted a hostile takeover of Take-Two prior to the release of Grand Theft Auto IV in 2008. The move was given the go-ahead by the Federal Trade Commission but ultimately talks fell apart by September 2008.
Activision made a major move when it merged with Vivendi Games in mid-2008, so a move like acquiring Take-Two wouldn't be completely unheard of for the company.
Take-Two is best known for Grand Theft Auto and being the owner of 2K Games, but it is also responsible for developing and publishing quite a few other series -- Red Dead, BioShock, NBA 2K, and Midnight Club among them. MCV points out Activision's sudden cancellation of True Crime: Hong Kong this week could be taken as a move made to benefit Grand Theft Auto, but that's pure speculation.
Assuming this rumor is true, it doesn't mean that GTA fans need to worry that the series will be turned into GTA of Warcraft Duty Hero and run into the ground as many would argue Guitar Hero was. (Keep in mind that three GTA games were released between October 2001 and October 2004.) It is speculated that the possibility of the next GTA being released in 2011, several key Rockstar employees -- Dan and Sam Houser, as well as Leslie Benzies -- have contracts expiring at the end of January 2012.
Were Activision to acquire Take-Two, it seems safe to assume that it would be willing to assure the Rockstar heads that they won't be forced to push out a GTA game on an annual basis. Activision showed with its Bungie publishing deal that it's willing to concede certain points in order to secure a major deal.
These are just rumors as of now.