While mana is still plentiful at level 80 -- you get enough regen just by the mandatory intellect and spirit on our gear -- it looks like things will be different in Cataclysm. We won't be able to just stack all throughput stats. We need to watch out for mana as well.
Rank 4 Healing Touch did some math that compares intellect versus spirit for mana regeneration. In short, at level 85, one point of spirit gives about 0.5 mp/5 and one point of intellect gives 0.90--almost twice as much!
What this means is that, as things stand, spirit is mediocre Always stack intellect in preference to spirit. You'll get better regen, and you'll also get spell power to improve throughput. Further, when comparing same-ilvl gear, always prefer the gear that has no spirit, no matter what else it has on it. Even if you need the regen, you can make up the lost regen by gemming half as much intellect as the spirit you gave up.
Personally, I find this a little disappointing, and I hope Blizzard rebalances the regen stats. I find it fun to balance regen versus throughput, and even to balance different kinds of throughput (crit, haste, mastery, spell power). On the regen side, it makes no sense that the one stat that is purely for regen is underpowered. It becomes one of those things where players who read the tool tips and follow basic logic end up playing worse than players who go snoop around on the Internet for obscure calculations.
It seems like all Blizzard would have to do is double the regen from spirit and halve the regen from replenishment. Then, intellect would be a so so regen stat but also give throughput, while spirit would be the place to go if you are really hurting for regen. Intellect would still win out for most people, but at least spirit wouldn't be so bad that people avoid gear with it.