It has been mentioned that one of the RPF people that has been working on the CM3 is also part of the group that developed the Slice TV box that uses the CM. Therefore, he has a vested interest in seeing that the CM3 can be a "drop in replacement" for the CM. In addition, one of the RPF engineers has posted that he has a CM3 to test. If I remember correctly, he mentioned using it in a pre-existing development carrier board (the sort sort in the dev kit with a CM).
On the whole, I think it is virtually a given that a CM3 will work in any board that uses a CM now. My one hope--and no one has said anything about it either way--is that the CM3 has enough eMMC to handle a full Jessie install (that is, at least 6GB and more likely 8GB).