I considered this myself, but decided against it because of healing spells; I couldn't figure out how to get them to work in a HP-based system like that.
Instead, I went with a Spell Point variant. Half the normal SP (as described in the DMG), but casters can still cast beyond that. However, every time a caster attempts to cast a spell with no or not enough SP, they have to make a Con Save or become fatigued. I think the DC was 8+SP cost of the spell. What I found with this system was the caster's never seem to have enough SP at low levels, but I was seriously worried about the crazy amounts of SP they get at higher levels.
After that I gave ALL casters d10 HD, but then at every level they have to choose how many of those Hit Points are Spell Points. The decision, once decided, is final. So you could have a first level spellcaster with 8 HP and only 2 SP, or the reverse, a crazy loon with only 2 HP but 8 SP. I like this variant so far, but am unsure how it scales. Again, that Fatigue system for casting without SP is in place.