Like others have said it's complicated and all mathy and stuff. The basic spirit-based regen formula is still this:
ManaRegen(SPI, INT, LEVEL) = (0.001+SPI*BASE_REGEN[LEVEL]*(INT^0.5))*5
If my understanding is correct (which it may not be), with each level having a different base rate of regen (and for the life of me I can't find a WotLK post with the rates!). Like Horse said though, until people start getting
replenishment (which you may actually have some classes getting close here in the next 10 levels or so) you can pretty much treat intellect as increasing your initial mana pool, and spirit improving your mana regen. Mana regen can come slowly though, even at 80 I think 1 point of spirit gives me about .37 mp5 or something like that, however we get tend to get a lot of it on gear (so much so we start trying to avoid it).
Once you reach the level cap a lot of the math is much better worked out, and there are programs and spreadsheets that can help you make decisions like this fairly easily, unfortunately there isn't much for people before the level cap, and the math isn't all that simple; for a video game at least.
Spirit will never be a bad option for a druid though. Druids were designed to work around spirit, and do so fairly well at all levels.
Edit:
Hmmm... think of it this way, spirit will give you more mana in the long run, but you need enough of an initial mana pool ( from increasing intellect) to get you to the point spirit is better. Spirit may give you more net mana after 90 seconds, but if you don't have enough initial mana to last that long (i.e. going OOM at 60 seconds), it won't matter. So ya, it's a bit of a balance thing as well... but spirit is awesome...
Edited, Jul 18th 2010 9:58am by someproteinguy