The SE main sites, not only the meta, would benefit from a crossposting capability available to users. Many postings on the main site here overlap with statistics, CS-theory, physics or Stackoverflow and crossposts would allow an exchange of answers across multiple sites, while on the meta many postings are of interest for SO or vice versa. In a setting that allows crossposting, migration is a special case of "editing the headers".
It would be interesting to know whether the SE 2.0 engine has crossposting functionality internally or at least is not incompatible with it as a future modification (as might be the case if the software strongly depends on the assumption that each question belongs to a unique site).
I see on meta.SO that this was discussed in several threads, but it was not clear whether anything has been implemented or what cross-posting mechanisms now exist.
[added: there is a new reverse cross-posting (multiple site cross-reading) feature called tag sets just now cited in the comments of the meta.SO thread. This does not provide cross posting but it might increase somewhat the set of users on site A who will see postings from site B. For the metas it allows reading a local meta together with (items with a specified set of tags from) the main SO meta as a single stream of postings.]