Blessing of Kings discusses Blizzard on Healers in Raids. Essentially it looks as though for 25s you will still need about 1/3 of the players to be healers. As Rohan points out, this doesn’t match at all with the number of people who are interested in healing.
Our little guild of about 10 has three people who had experience raid healing before we rerolled. All three of us rolled non-healing classes. Two of the former healers have toons we’re willing to heal with, but it’s definitely not my first choice of spec. From what I can tell, it’s the same thing for the other guild in our alliance – people will heal but they’d prefer other roles.
Warcraft is a game – there’s only so long that people are willing to play a less-fun role even if it’s an essential one. Personally I expect to burn out on Wrath raiding much more quickly than I did on Burning Crusade raiding. The scramble to find people to fill out the needed roles is a pain and I don’t want to do it anymore.