Hi Tim,
As weinstein5 said if you upgrade your vCenter to 5.1 it can manage both clusters running different versions. You can also vmotion/svmotion VM's in different clusters within the same datacenter. However, there are some caveats :-) I recently did a similar migration although it was between 4.1 and 5.0. The main issue I hit was the differences in the EVC mode on the clusters. Unfortunately the old cluster had been set to a level that wasn't compatible on the new hosts which meant that all of the VM's had to be shut down. Check your EVC mode first, hopefully your new cluster can be configured to the same EVC mode, at least for the migration and upgraded later.
Other than that if both clusters have the same storage presented, etc, you should be fine. Oh, also check your vswitches between clusters, particularly if migrating from standard to vDS in one hit, you need to juggle this carefully :-)
Cheers,
Ben Loveday