Hi Mike,
There's a SFDC Integration Guide for Marketo on this site. Just look for it.
I also wrote a guide to that that you can download: marketingrockstarguides.com
Some issues to prepare for:
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make sure SFDC is as clean as possible. this includes workflow and dedupe. You can use Marketo to clean up some data values later.
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Do the sync over the weekend (esp if you have lots of users).
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Plan out data value migrations.
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Plan out Scoring methodology based on historical data if you can. Choose scores and workflows ahead of time. Test it if you can.
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Email Migration - make sure you prepare a subs management workflow plan and an email prefs migration from any older systems or in SFDC.