There are many cases out there that support VESA mounting, so you could easily get a Mini-ITX system behind a TV, and then mount the computer to a wall VESA mount or VESA monitor stand.
Not sure I can think of a TV that has enough space in the back for a computer. Not without using laptop parts, like the aforementioned All-In-Ones. If you took the case off the back of a TV it would be rather exposed.