As part of the large Cataclysm information released tonight, we've gotten quite a bit more information on how raids will be changing. One exciting idea is "downshifting," namely, the ability to split a 25-man raid up into separate 10-mans. There won't be any sort of "upshifting" allowed, however. Simply put, if your 25-man fails to proceed due to numbers, it can break up into separate 10-mans with the same bosses defeated as the 25 was at the time of division, and if you later get those other players back, another 10-man can be created with the same conditions.
In addition, raid lockouts will be more forgiving; as long as you have killed the same bosses as the raid in question, you'll be able to join a raid in progress even if you've been in a different raid to the same instance during the lockout. Basically, you will not be able to kill the same boss twice during a lockout, but you won't be totally cut off from raiding with others simply because you were in a different raid lockout earlier that fell apart, as one example.