Speaking in an interview published in the latest edition of http://www.edge-online.com/news/dirt-3-confirmed, Codemasters' Vice President Gavin Chesire directly refers to DiRT 3.
Here's the relevant section, to quote:
"Really, it's all about letting the teams just innovate. There's some great stuff coming from Birmingham that's going into DiRT 3".
He continued: "There's an overwhelming arcing vision to all of our racing titles, and GRID is incredibly strong, but we're lucky enough to be in a position where we've got racing games in different areas.
DiRT was a complete reboot - we threw away what was going on and redid it completely - and then we rebooted GRID because it wasn't top gun, and then we rebooted DiRT 2.
I'm not saying we're throwing all the codebase away, but we're throwing the thoughts away and coming out with something fresh and new. Because if you don't innovate, you're dead."