Merrick here...

Many different sites are reporting snippets of information from Guillermo del Toro's recent appearance on BBC Radio's Simon Mayo Show, but the most comprehensive write-up I found came from G4 (HERE). If you know of a more detailed breakdown of Giuillermo's appearance, please tell the world via the Talkbacks below.

In short, during the Simon Mayo show, Guilermo talked a bit about THE HOBBIT. He said Andy Serkis, Ian McKellen, and Hugo Weaving will reprise the roles they originated in "the trilogy". This has been assumed/presumed for some time, but now I guess it's official.

del Toro also mentioned that...

the film will take some liberties with sub-plot material that is briefly discussed in the book. Most notably among this, is Gandalf's departure from Bilbo and the Dwarves to confer with The White Council in Dol Guldur on dealing with an entity called the Necromancer (which later turns out to be Sauron himself.) We've always suspected this based on some statements, but this time, the intention to document Gandalf's quest (which is not covered greatly in the book) was articulated clearly.
[EDIT]
Smaug the Dragon, del Toro revealed that after eight whole months of design work, they have only just “cracked the basic engineering.” It is anticipated that another six or seven months of application will be needed before the great flying, fire-breathing, gold smuggler will be in any kind of presentable form.

All per THIS article at G4.