Alison:
We brought as much as we could over from one instance into the other. We chose to use the instance that was already connected to SFDC as the primary. That meant we had to go through and indentify each and every program that we wanted to move from A into B. Anything not in a program (which is all that can be exported from Marketo) had to be downloaded externally or completely rebuilt in the new instance. You also need to build membership lists for all programs so that you can run campaigns in the new instance to rebuild membership and status for any of the data you want to maintain/keep. We used a third party, in our case LeadMD, to manage the export and import function. This is a large task and, depending on your manpower, can be easier using a vendor to complete.
We spent a ton of time internally conversing about processes, set up and structure before and during the integration. It still wasn't enough. I'm amazed about how much I thought we had come to agreement on that still isn't set up completely. LeadMD's efforts to import the programs was really fairly easy compared to the ongoing process of integrating two very different marketing organization styles and processes. As with almost any integarion, the hardest part is establishing the processes and getting agreement among all parties.
Let me know if there specific areas you'd like to know more about.