While this is disappointing news, it's probably a good call on Sacha Baron Cohen's part. With the success of both characters, I can't imagine he'd be able to fool many more people with his tomfoolery (although I was always surprised that he was able to dupe so many people to begin with).
Every time I watch “Borat: Cultural Learnings of America for Make Benefit Glorious Nation of Kazakhstan” or "Da Ali G Show", I'm amazed that Cohen is able to stay in character with some of the ridiculous shit that comes out of his mouth. No laughing, no snickering... hell, he doesn't even crack a little smile. There is no way I'd be able to pull that off.
In his interview with The Daily Telegraph, Cohen said that he would sometimes be in character for 14 hours a day. I guess he's going to have a lot of time on his hands now - more time to spend with hottie girlfriend Isla Fisher, I guess... tough life.