I think they should do it for 2010 because:
1) Bale is beginning to look old now (hes gonna be 36 if they film it in 2010 for 2011 release)
2) Marvel has 2 big movies coming out in 2011 (Avengers and Captain America) and Iron Man 2 is already set for April 30 2010, so they can get that July spot again for 2010 and not have to compete with Marvel.
3) Batman 3 will receive more momentum from TDK being released in 2 years rather than 3 or 4.
4) There is no reason for Warner Brothers to delay or not believe in a third movie with Chris Nolan as director/writer, so development and funding should be quicker this time.
5) Nolan, his brother, and Goyer can write a great Batman movie regardless of time, and the Nolan's don't have another movie to do (yet) like The Prestige in between Batman movies
6) Bale has no filming projects, for now at least, from summer 09 onwards, so WB can ask him leave that space open for Batman 3 filming.
7) Spider-man 2 was a great sequel and great drama/action movie, and was the best superhero movie ever until TDK. It was released only 2 years after its predecessor.
Does that mean Nolan isn't doing The Prisoner anymore?
I read he just helped write that film, but that he's probably not directing it since its similar to his first films.