Not the most fun I have had in a long time but here is a quick rundown.
I have a 8 Node Multirole stretched DAG with a witness server in a third datacenter.
To perform the SP1 installations I followed the following steps:
1. Fail over all servers in Datacentre B and ensure all services are fully operational from Datacentre A.
2. Uninstall the Exchange UM language Packs from the previous CU3 release, followed by a reboot.
3. Initiate installation of SP1 on all passive servers followed by another reboot.
4. Install the SP1 UM language packs followed by another reboot.
5. Fail over all databases and services to Datacentre B and repeat the process.
Whilst everything went rather smooth with no service interruption the SP1 installation took about 8 hours in my environment, which could potentially be reduced.
Performing the installation using the unattended option from the command line does greatly reduce execution times.