When brides come into my shop who have weddings a long way out (over a year for example), I always fully explain the process. It usually takes 4-7 months for a dress to be ordered and you want an additional 6-8 weeks for alterations. So 8 months is not out of the question for ordering....
Most important is that the bridal shops have NO CONTROL over when a dress will be discontinued - the manufacturers do that. The manufacturers also do not give us any advance notice when they decide to discontinue, just a week or so. Sometimes the dress isn't selling well (nationally) and then they pull it. Or, perhaps the special lace, trim, fabric (whatever) is no longer available so they can't make any more. Whatever their reason, it just happens!
So, if there is a special dress that you have your heart set on, by all means just order it!! You wouldn't want to get all excited

, put your dress up as your screensaver and everything, then go back to the store in 4 months and it's not there!! We store the dresses at my shop (many make you take them home when they come in) and last night in the storage area I saw a dress for October of 2007, must have been ordered in the summer of 2005. It's never too early when it is "the one", plus that is one more thing to check off on that giant wedding to-do list.

Good luck in your decision making!
Oh, and don't worry about the sizing. Tell your consultant that you want to actually see the manufacturer's sizing chart. Have her take your measurements and do a thorough comparison. Order the size that YOU WANT based on your measurments and the chart. Remember that you will have alterations done and it is easier to alter a dress smaller than to open the seams and make it bigger. But ultimately order what you want!! Feel free to PM me when you figure out which one you want and I can help talk you through the sizing issue!!