Holy Pally and Resto Shaman would be powerful but you will have Mobility Issues as one of the posters above have Stated already. The pally Mastery will be nice for Mitigating Damage and saving heal time but unfortunately it wont be on the same level as a Disc Priest. Currently we run Disc/Shaman. Its a very nice combination and it allows us the mobility we require by having shields instead of hots. Unlike the poster above me who, i dont know if they do play a shaman or not, was mistaken we have an emergency heal (Healing Surge) which although mana inefficient is a nice .9 - 1.1 Second cast which is very nice its also rather large. Basara Is correct that Paladins will be very mobile in Cata but that puts a lot of stress on them if they are the only mobile Healer. So its all up to how you feel your group can work.
Basically running a Pally/Shaman combo would work, ive seen the Beta Raids and from what i can tell there would be many advantages to this combo. I run from time to time with a Pally/Shaman and even during ICC Hardmode Progression we were able to 2 heal even the most difficult of fights. When we are talking Cata Progression you might need the 3rd healer just until everyone learns the fights and gets out of the starting raid blues. Shaman/Pally/Anything works well.
Also, Druids have lost a lot of their ability to spam hots so they will be less Raid Healers and focus more on a target by Target basis. While they are great raid healers its not the same as it was in Wrath, just like everything has changed since wrath so you cant judge them or any Healing class too harshly until we hit 85.
dont need CDs when you have awesome tanks and healers :D
Bad, Just. No. I hope this was a joke and i shall give you the benefit of the doubt.
If you have anymore Questions or if you raid leader wishes to debate it because im sure they will. You can find me Horde Side on Lightninghoof Always open for a chat.