It was bound to happen some time and now according to KC Lemson (the release manager for Exchange) Exchange 12 will ship on a single DVD:
I wanted to let you know how important your feedback was to us making this decision as well as address some of the concerns that were raised on the anti-DVD side. Another plus of this experience was it was a great proof of concept as to how useful this blog is in gathering diverse customer feedback, so thanks for all the comments and please keep them coming. Even when we don’t explicitly ask for feedback, the comments are still useful, any time you tell us what you think about something, positive or negative, we appreciate it.
First, I should explain what would be taking up this much space. The main feature is unified messaging, which is a core feature in E12 that we’re very excited about bringing to the masses (and trust me on this one, there are some amazing features in UM). The largest part of this feature are the text-to-speech engines that are needed for each language. These engines allow you to call up your server on your phone and have it read your email and appointments over the phone to you. We estimate that the final size of each language of these engines (using a non-lossy compression) will be around a couple hundred megs and we are working on including as many languages as we can.
That would do it. More on all the considerations that went into the decision by following the link.