From my experience (and disers please feel free to correct me if I'm wrong) is that you can contact Disney to discuss your wedding 18 months out. You can call them earlier but you will only be able to speak to an assistant who will pretty much direct you to the wishbook and website. I've been engaged since Nov 2006 and when I called prior to 18 months out thats what happened to me.
As for answering your individual questions for your wedding, at 18 months out you can call and schedule a site visit and then you will be assigned a coordinator. This is when my coordinator starting to answer my specific questions (what time will this happen, how do we work this). I just visited Disney last week (I was at 1 year and 1 week out at the time of site visit) and now I'm waiting to hear from my coordinator about my official wedding date.
They are very strict about the one year policy, or at least in my experience. My 1 year mark fell on a Sunday before Labor day and I had to wait until after the Holiday even though my date was closer to the Friday before. I still had to wait until my one year mark passed.
The only exceptions I can see is sometimes international brides (though it varies, there is no standard) and Couture Weddings. I believe International brides may be able to have there individual questions answered sooner by a coordinator, but not book until 1 year.
I think Couture Weddings may be able to book more than 1 year, but they also use the Contemporary resort for their reception instead of the wishes locations
HTH!