We do have Siebel as a CRM, but we don't have an integeration. When we were exploring the integration one of the major drawback was relatively weak API support on Marketo side, Marketo doesn't have any API's for programs, so you can't programmatically link Marketo programs to Siebel campaigns.
Note: We use the Siebel campaign ID as Marketo program name [manual input] to enable loose integration.
I would envision a loose integration where you send Marketing Qualified leads from Marketo to Siebel and bring the opportunity information back into Marketo from Siebel.
There are some middleware's like Boomi that might make the integration little easy.
http://www.boomi.com/news_and_events/press_releases/090109
Hope that was helpful.
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