In my office, we had a meeting where the project manager suggested that the wireframes were not the final layout. In theory, while I think this is somewhat true, in that the UI designer might modify the appearance of some elements, in practice I strongly disagree that major layout changes are OK. If there are wireframes, it implies a semi-final layout that takes into account certain functionality. It is generally true that wireframes are reflective of layout, or not?
ETA: Just to clarify, this is specifically in instances where there has been a designated UX portion of the project and wireframes are a key deliverable, and presented to a client, as opposed to wireframes that are created for the sake of brainstorming or "putting an idea down on paper" that the client does not see.