Do they stack? I've been maintaining my stance that they do not stack vs another mage who says they do stack as it would be stupid if they didn't. My position comes from back in the day when they didn't stack (unless you pulled the trick of having mage armour up while respeccing into arcane meditation as as soon as you lost mage armour you couldn't regain the stacking short of a respec) I just haven't been bothered to look into the subject and I don't remember seeing anything in the patch notes about it...
It should also be noted that Pyromaniac in the Fire tree is getting a 10/20/30% mana regen from spirit active while casting effect during the next patch as well.
Since both Pyromaniac and Arcane Meditation are reachable within the same spec, it is actually possible to reach 100% mana regen from spirit active while casting.
Note though that Glyphed Mage Armor, 3/3 Arcane Meditation and 3/3 Pyromaniac is actually 110% mana regen while casting, but the game cannot go over 100%, so you should limit yourself to 2/3 Arcane Meditation and 3/3 Pyromaniac, should you choose to spec this way.