I think we can agree that a disc priest can have a spot in a 25 man. I also won't argue that some disc priests can heal in a 10 man, but I will contest that it does not speak for the vast majority. I think disc priests are going to be in the same situation as balance druids were in BC. They offer nice raid buffs, but they need to pull there weight in raids as well. The changes to the discipline tree will make it easier for them to do so, but I don't believe it will be an understood fact that all disc priests will be able to keep up. As a raid leader, I would definately be interested in trying a disc priest in a raid, but I would probably ask a proven holy priest to try it first, before I specifically recruited for a discipline priest. My suggestion to someone who wants to raid discipline would be to apply as holy, and prove that you can heal, then suggest to try raiding discipline. If in fact you can perform as well, if not better, then any raid leader should allow you to keep your spec. This is what nearly every class has had to do at some point.
P.S. You cant fill in for a holy pally because you don't have pally buffs. Yes you have spirit, but it doesn't substitute for might, or wisdom, or kings. Sorry. A discipline priest will fill a priest spot.