I tend to define a mockup as a functional analysis made fancy by wrapping a visual coat around it so everybody can understand the logic and structure featured in the proposal.
For us (UX, IxD, designers, developers, industry professionals...) it's a clear and understandable document shaping the foundation of the flow, so it is important to get the client to understand (and approve) it. This is where I tend to experience some 'problems'. Some clients (certainly not all of them!) interpret mockups as a design proposal and instead of approving functional features they often comment on placement, design and even the dummy text... yes, they do.
Whenever we can we also create a provisional design (in support of the mockup), but in most of the cases the lack of feedback (and time) doesn't allow the creation of a graphic counterpart.
How do you people avoid this problem?