Who should be the IT owner (signatory) of the SLA and manage the Customer relationships?
As far as who should be the IT owner (signatory) of the SLA, remember that SLAs are not between functional groups and the IT users. It is between the IT organization as a whole and the IT users. With this understanding, the signatory on the IT side should be the person in the organization who is fully accountable for the entire IT organization. This is typically a CIO in a large organization or a Director of IT in a smaller company.
Who should manage the relationships is a completely different question. The person who manages the relationship has to have the technical knowledge of an IT professional and the business knowledge of a MBA. OK, maybe not that much business knowledge, but they must be very comfortable talking to the business in business terms. They must then translate those business requests into something that is actionable by the IT organization.
If you find a good Service Level Manager who is able to negotiate with both the IT and business groups, then pay them well and make sure their efforts are recognized. They are hard to come by.