Really nice post from MSExchangeTeam explaining the Exchange 2010 version-based routing. Long story short, Exchange 2010 Hub will communicate only with a Mailbox Exchange 2010, if you have an Exchange Server 2007 Mailbox a Hub 2007 is required to communicate with the legacy mailbox.
By now, you’ve seen a few of our Spotlight on Exchange 2010 postings. If you have, hopefully you’re excited about the great work that has been going on to make Exchange 2010 the best version of Exchange yet. So then the practical side of you kicks in and starts to wonder what it is going to take to start utilizing these new features when the time comes. Certainly, rolling out a new version of Exchange is rarely an endeavor to be entered into lightly.
In the interest of making certain that you can begin planning early and as informed as possible, I want to call out a requirement that we had to put in place for Transport server roles in order to deliver a quality product with cool new features. This requirement is often referred to as “version-based routing” or just “versioned routing.”
What this requirement means
Simply put, this requirement means that Exchange 2010 Hub Transport servers and Exchange 2010 Mailbox servers will only communicate with each other. Also, it means that Exchange 2007 Hub Transport servers and Exchange 2007 mailbox servers will only communicate with each other. However, any version Hub is able to communicate (via SMTP) to any other Hub server. So, for a user on Exchange 2007 to be able to send to a user on Exchange 2010 in the same site, the mail must pass through both Hub servers.
Read more at source: http://msexchangeteam.com/archive/2009/10/14/452859.aspx