Microsoft has released Service Pack 3 for Exchange Server 2010 and Update RollUp 10 for Exchange Server 2007 SP3 today.

These updates will make your existing Exchange 2010 and 2007 environment to coexist with Exchange Server 2013.
Exchange Server 2010:
Microsoft Exchange Server 2010 Service Pack 3
http://www.microsoft.com/en-us/download/details.aspx?id=36768

Issues that the service pack resolves
http://support.microsoft.com/kb/2808208

Exchange Server 2007:
Update Rollup 10 for Exchange Server 2007 Service Pack 3
http://www.microsoft.com/en-us/download/details.aspx?id=36708

Issue that the update rollup resolves
http://support.microsoft.com/kb/2788321

Note: Microsoft also released Update Rollup 6 for Exchange Server 2010 SP2.
Update Rollup 6 for Exchange Server 2010 Service Pack 2
http://www.microsoft.com/en-us/download/details.aspx?id=36716

Issue that the update rollup resolves
http://support.microsoft.com/?kbid=2746164

I would strongly suggest not to install SP3 for Exchange 2010 right now, instead play around with RU6 in the LAB first and then install it in the production. Do the same for SP3 as well.
Install the SP3 after enough testing in the LAB.

We all have been waiting for these release since few months and finally Microsoft has delivered it. Now, this is were the FUN begins.

Update: 15-02-2013
We still can't coexist exchange server 2010 SP3 with Exchange Server 2013 entirely. Cumulative update (CU) is required for Exchange 2013 for coexistence between Exchange 2010 SP3 and Exchange 2013. The change to the cumulative update model, from the past rollup update model, was just announced last Friday.

We should expect the CU for Exchange Server 2013 in the month of April or late March. Let's keep the finger cross.