Update 1/14/13: We have updated this blog post to narrow the release timeframe from "first half" to "first quarter" of CY 2013, as announced in Exchange Server 2013 Reaches General Availability.
...
Updated Jul 01, 2019
Version 2.0
Blog Post
@Simon Jones: The page http://technet.microsoft.com/en-us/library/aa996719(v=exchg.150).aspx
technet.microsoft.com/.../aa996719(v=exchg.150).aspx states Exchange 2013 Preview cannot coexist with 2003/2007/2010, which is still correct as the Preview version was only meant to be tried out in a lab by itself.
While still subject to change until RTM is declared; at this time only Exchange 2013 RTM will have support for co-existence with 2007/2010. The legacy versions will have to be at certain versions for coexistence to be supported, and this blog article states
Exchange 2010 will require SP3 for co-existence with Exchange 2013 RTM.
To summarize at this time; Exchange 2013 Preview supports no co-existence while 2013 RTM will (unless things change before RTM) as long as other pre-reqs are in place.
This should become more clear once RTM happens and TechNet is refreshed for RTM, but we'll see if we can make it a little more obvious in the current documents.