Exchange 2013 went RTM\GOLD yesterday on 10/11/12 day 
For the official announcement check out the EHLO blog post: http://blogs.technet.com/b/exchange/archive/2012/10/11/the-new-exchange-reaches-rtm.aspx
This blog post contains 15+ links to articles that the Exchange team has created over the last several months on Exchange 2013.
For my summary on what’s New & Cool in Exchange 2013 see my blog post here: http://blog.jasonsherry.net/2012/07/31/new_cool_exchange_2013/
Volume licensing customers should be able to download it in mid-November. In addition, Office, SharePoint, and Lync 2013 have also gone gold\RTM. See the Office team’s blog post on that here: http://blogs.office.com/b/office-news/archive/2012/10/11/office-reaches-rtm.aspx
But for those clients using Exchange 2010 you will need to wait until “the first half of calendar year 2013″ since Exchange 2010 SP3 is required to co-exist with Exchange 2013 but SP3 won’t be released until then. #FAIL 
See this EHLO blog post on Exchange 2010 SP3: http://blogs.technet.com/b/exchange/archive/2012/09/25/announcing-exchange-2010-service-pack-3.aspx. Luckily I have access to the beta version of SP3 and will install it, upgrade to 2013, and then retire 2010 before SP3 goes gold\RTM.
Is that 2010 SP3 beta public?
Per: http://blogs.technet.com/b/exchange/archive/2012/09/25/announcing-exchange-2010-service-pack-3.aspx
1st half of 2013
The Exchange community is disappointed it’s not out now.
I haven’t heard of a public beta either, only TAP & MVPs have access that I know of.
anyway to get access to a copy of this service pack
Only Exchange TAP (Technology Adoption Program), MVPs, & MCAs have access to the beta currently. All of us are under NDAs and can’t share the bits or info about them, until MS makes them public.