Mixed-core-architecture chips are common in embedded and other special applications – think the Sony Cell. Usually the cores have dedicated functions – for example, one drives your friendly local nuke plant, and one drive the UI, so process migration is not an issue. The cores need not even be from the same vendor nor be of related architectures. Crimson is a testbed for such use in our specification system.
Process migration across dissimilar cores has been the subject of some academic work. It appears to be practical in a JIT environment, although utility, except for reliability reasons, is questionable.