If I had to do some guessing based on Didio's usual (opaque) answers I'd say Ollie becomes a wanted man into hiding and Dinah goes to team up with her pals to help him (hence the big GA story Didio has been promising for 2010 - Green Arrow: Wanted Dead or Alive or something like that.
As for Zatanna, the April date could just be a coincidence as to when they have enough Roux artwork ready but the minute I read I couldn't help but also think that the Search/Return of Bruce Wayne is rumored to be in Summer 2010 and that Dini is also a major player in the Bat-books (and a huge Bruce/Zee shipper). I can't but think Zatanna will play a role of some kind in Mr. Wayne's return.
I am getting a little tired of all the equivocating over the Marvel Family though. It's not like something big is going to happen to them in 2010 (you never hear either Batsons named in the big plans Didio is always talking about for 2010) so just let us know if they'll be showing up after the PoS one-shot. I can't forget that several months back Didio also said Billy and Mary were going to appear in several DC books in the coming months (but wouldn't name them) and now several months later - nada.
I admit I also thought Ivan Reis was going to handle a new Aquaman book with Johns. Geoff has already said he has big plans for Mera and Black Manta and I would be hugely surprised if both Arthur and Tempest weren't back at the end of BN. So how many books is Johns writing in 2010? Flash, GL, possible JLA or Aquaman book (with Gary Frank?) and the Reis book. This in addition to writing the Flash and Shazam screenplays?