Hi there,
I have worked on this a couple of times with clients.
A few important rules of thumb:
1. If you can just rebuild in Marketo ... do so. "Migrating" is far harder than just re-building
2. Activity information like historical Click links in Emails etc are a manual migration. The Activity histories will not carry over. If there is any critical information you will need to carry over in terms of behavior, you will need to do thiese in custom fields.
3. Start Your Scoring Model Over, you could always migrate over the final score, but you will not be able to bring over the whole model real time
4. Anyhistorical program/ cmpaign performance you might want could be summarized and extracted in reports out of Eloqua.
5. Data - Make sure you don't miss bringing over your supression list.
6. Make sure during the migration both systems are not writing into your one instance of salesforce. You run the risk of overwriting data.
7. Make sure your system is prepped prior to migration (i.e. you have your fields in place etc)
There is a lot more, but that was a few that came to mind!
Good luck!
Maneeza