You briefly mentioned governance, how critical is it that the organization has a cross Biz Unit Enterprise Architecture body to enable (publish) access to a data silo(s) somewhere within the enterprise?

The answer differs based on the size of the organization, but we certainly think that an EA body can show a great deal of commitment to a properly-planned adoption/implementation strategy for mashups.  To enable cross-business unit mashups, you typically plug-in to the existing centralized identity management system, such as LDAP. This allows you to set governance policies and propagate the user’s credentials to the services.