We have successfully set this up as of last week. Previously all 2003 SP2 domain controllers and servers, running Exchange 2003. We added one brand new 2008 R2 domain controller and all went well. The only thing we ran into was a problem with activesync users not being able to get their email after the upgrade. We've had that happen one other time in the past, so I can't confirm that the 2008 DC was the cause, but certainly coincided with the timing of adding it.

If you run into that issue, Microsoft KB article 817379 is what we used to restore mobile connectivity.

Otherwise, things have been smooth. I definitely would not upgrade the functional level of the domain until you have upgraded to a newer version of Exchange, though.