Resto Shaman: Spirit Vs. Haste (4.0.1)

There is no point where you are "safe" - the amount of spirit you need depends on the way you heal and how well your group does (DPS standing in fires drain a lot of mana ;)). As long as you feel comfortable with your mana, go for haste, I guess you won't replace all of your gear at once, so you can gradually ease into it. Mana is well? Go for more pieces with haste and reforge spirit into haste. Mana getting tight? Reverse these reforgings, maybe keep alternate equipment with you.

But right now, ICC has become ridiculously easy for many people and if you want a clear decision now, I'd say: go for haste right now, you will have enough manaregen as it is.

This will also depend on your mana pool, as well as how many healers you run- and Nayo is exactly correct in that it is very dependent on how you heal and if you use your mana tide totem, etc. Your glyphs can factor in as well for mana regen. This is a type of touchy/feely point currently. I'm sure that will be changing greatly once Cata is live - and we have Cata content to deal with. Matticus has some excellent looks at what it takes in the amount of spirit and mana pool to heal a 5 man versus a heroic 5 man, versus raid content- in terms of spirit - http://wow.joystiq.com/2010/11/05/raid-rx-in-the-spirit-of-regeneration/#continued

This is a good look at the future but for now, pre Cata, most of this is dependent on what you feel comfortable at.