I don't know why they wouldn't just assign Geoff Johns to oversee all the DC movie, especially if the plan is to make all the movies tied-up in one universe like Marvel is doing. Seriously, there's no one else better at this stuff than Johns. He understands what makes the DCU tick. Heck, why not give him a crack at the Superman movie relaunch?
It would certainly be cool if he contributed to the GL script. I remember reading interviews with Guggenheim on Newsarama.com where he touched on the subject of "Secret Origins", but I'm not sure if he said notes where taken directly from Johns. What I am certain about is that John August mentioned in his blog that he took some pointers from Geoff Johns for his now shelved Shazam script.
Anyways, I am overly excited about this movie - it's about time! Gotta say, though, I didn't expect someone like Martin Campbell to tackle the project. I know that many people don't give him enough credit, but personally, I enjoyed most of his stuff - especially Mask of Zorro. What worries me about him is that I don't know how much he is into the Green Lantern lore. He's never done sci-fi before, which is what GL is all about. Is he aware of the huge fans' expectations? Would he treat it with the utmost respect that it deserves? Only time will tell.