There's nothing "behind the scenes" going on, in the sense that it's not hidden. What's holding this project up are two issues.

First, MGM and Warner Bros. haven't ironed out all the details to their financing arrangement. This really hit a snag when MGM started having all those troubles. However, as far as I know, that's all but taken care of now due to Spyglass and others. So now Warner Bros and MGM (who are both co-financing the film) are just ironing out the details of who pays what. Plus they need to make sure MGM will be able to pay their debt (which is part of the whole Spyglass/MGM thing) before they can greenlight.

All of that is going down in Hollywood. Then across seas in New Zealand you have a whole other beast blocking the project. The New Zealand actors are wanting to start up some unionization. There's this whole Hobbit Producers-vs-New Zealand Actors thing going on. Can't entirely remember the details about what set it off, but suffice to say the actors want to get paid more and want to unionize. However, this isn't legal for them to do under New Zealand law as they're "independent contractors" rather than employees. This has caused a standoff in which all the New Zealand actors are doing this whole "Do Not Work" order against Hobbit producers. If I'm not mistaken, SAG and others have got behind them as well. There's a movement to try to get the actors considered as employees rather than independent contractors, but New Zealand law makes that a tricky thing to do as well.

Of course they're fighting hard to try to fix that situation because as long as Hobbit producers end up signing an agreement with SAG, they won't be in any violation against SAG and support for the New Zealand actors guild may crumble. Especially given that producers and studios behind Hobbit are looking to move to another country to film. Hobbit studios and producers believing this to not be their problem per se. Rather something that the NZ actor's guild should be taking up with the NZ government and NZ producers guild instead of singling out and placing it all on the Hobbit producers.

I didn't know anything about the New Zealand actors thing. I don't understand why they don't just stick with American actors then?

As for the other business issues, I don't really see it as an excuse either way. I mean yes they are negotiating and what not, thats understandable. I don't know how much of a powerhouse WB is and what not, but look at all the other films that went ahead and got made while problems were persuing. There were issues with Watchmen left and right. WB said they had rights since they own DC Comics, and Fox said they had rights since they purchased them from DC before they were sold etc....Yet they sue each other after release, and its done.

From what I understand, American actor guilds are also supporting this NZ guild. Which means if an actor's guild is against it, they can't work the movie as their guild has a Do Not Work order against the movie. However, I think this is an issue that's more or less being allowed to be drawn out in order to find a suitable solution for all parties. This being done until MGM and Warner Bros get everything squared away. And it can't go forward if the two studios don't say so yet because they fund the project. Granted other films have, but that's at the studio's discretion. Basically, the way I see it is that Warner Bros is waiting to make sure MGM pulls through before greenlighting the project so they're not stuck with the whole bill.