It works but it is not supported. I am currently moving more than 2000 mailboxes to Exchange 2013 with mailbox + CAS roles on separate server but coexisting in the same forest/domain.
Mailbox migration works when initiated from the 2013 side. I had a lot of problems to work through to get it working but finally managed it.
i came from ex2007, i guess its the same for 2010 but this is only my estimate.
first update to sp3, a lot of troubles, was a cas array first, one node removed, the sp3 setup didnt work quite well, but i guess you guys manage to update your servers :P
after that, two new servers, 2012 servers. domain function level to 2008 r2, forest level 2003.
now you may start the setup for exchange 2013 after installing UCMA and stuff, setup will NOT give you an error regarding the old exchange in the organization.
now comes the weird part:
in the ex mgmt shell on the new servers, i got some errors when creating new databases, version mismatch etc. If you create the database from the webgui with the assistant, just name it Test, dont mount it, leave it at the default path - it will be created successfully. thereafter, you may rename and relocate the databases.
i also had troubles with public folders migration, didnt work out, i saved everything from the old public folders to a pst and deleted them, the database and everything. had to do it in adsiedit because it wouldnt remove otherways because again of a version mismatch.
then you may configure everything else, autodiscover, connectors and stuff, works flawlessly.
now move offline adressbooks and the mailboxes, thats it. just have to start it from the 2013 web gui, but this is normal behavior.
why i did it, well, first i read in the internets that from ru9, exchange 2013 migration will be supported, this was a wrong info because the update needed is not yet out. i already got most thing up by this time, also hybrid is needed and with exchange 2013 you dont need a dedicated hybrid server.
if you have any further questions, ask.