Mike--I always understood that "enterprise architecture" was invented by John Zachman, then at IBM working on business systems planning, in the 1980s.
Regarding evidence and SLA's -- where there is a contract (or a "public offering" of a product or services with associated claims of functionality and performance) there is, in principle, legal enforceability. But most business buyers take account of whether or not the offeror is a "responsible source", which means a track record of some sort and enough net worth to allow the buyer to actually collect something via the legal system,
I believe Ken and I have similar experiences with services offered "internally" (within some organizational boundary) where formal contracts don't apply (Division A can't sue Division B). In that case, Division A would like to have some assurances that Division B will perform, or some more ad-hoc recourse (like the Manager of Div B's head, for example.)
I think most commercial IT services will be offered on a "public offer" basis--same terms for all or most customers, which would be public. In fact, we often also see offerors of all types citing their own or a third-party's statistics on performance, etc. So certainly there is evidence available to buyers in most cases.
Regarding Ken's story of an individual trying to get resources from a large company: that's an interesting observations. Maybe we ought to formally recognize the role of "relative market power" in modeling shared services.