We started with 6 2003 server domain controllers and have slowly been migrating to 2008 R2. Currently changed out 4 of the 2003 to 2008 R2. We have been runing this way for over a year without any problems. We now have the PDC on the Server 2008 R2. We plan to replace the last two 2003 controllers this summer and go Native 2008 R2.
There should not be an issue to run 2008r2 as a member server. If you want a 2008r2 DC then there are a few more things to watch out for.