Today Bornakk, a blue community poster, said that right now all Blizzard intentionally wants is to have two specs be available, but that they "...want to see how the system works out first," before considering triple specs. I think it's pretty safe to read between the lines here and take it to mean that it's possible and has at least been discussed by Blizzard to some extent if community people (and not a developer like Ghostcrawler) are already talking about it. Remember here that community folks usually don't talk about development related things without running it by or hearing it from a developer first.
Triple specs would offer even more flexibility to play the class in whatever way is possible. A druid would be able to switch between a healer, tank, and caster role in a single instance / raid. A paladin would have the same amazing flexibility. While each class right now has at least two specs that can really shine in specific situations, triple specs would allow three-role hybrids to become amazingly powerful characters.