We’ve now heard from multiple sources (once at Westeros and also in the comments of my previous post) that auditions for many of the roles are currently underway. It appears that all of the roles they are currently auditioning are that of the children characters. Dany and Bran have been specifically mentioned by the two sources. Also it was mentioned by the anonymous commenter that a final decision won’t be coming until the end of June. It seems they are going to be taking their time and making sure they are getting this right.
While open auditions seemed like an inevitability for the younger characters, I am curious to find out who they have lined up to play the key adult roles. Ned, Catelyn, Jaime, Cersei and Robert all need to be filled soon and will likely be filled with experienced actors. I suspect they have to be in negotiations with at least a few actors for these roles.
Either way casting seems to be in full swing and within the next couple months we should have most of the cast finalized.
Update: According to the anonymous commenter, the other roles currently in audition are Sansa, Arya and Joffrey. They are also auditioning for the roles of Robb and Jon at this time, according to a poster at the Westeros forums.