I agree with Sagittaire (erm, gamecat) that it could easily be a normal magical item. After all, there's an Epic item that grants immunity to all elements.
It would threaten the Cleric though. Suddenly, you wouldn't require a Cleric since the Wizard could take over the healing duty in the group by using Fireballs and Meteor Storms. Unlike the Cleric, he could heal from a mile away.
I think that simple Cold Vulnerability would offset the item insufficiently (you would just remove it when Cold monsters approached, since it is rare for monsters to have both Cold & Fire attacks anyways).
There's a 5th-level cleric spell in the Book of Exalted Deeds that absorbs 10 points of energy damage per level (choose type) like Protection from Energy, and heals 10% of the damage absorbed.
That could be used to model it after. You might want to notice that it is a Cleric spell. I presume that WoTC wants to keep anything with healing power out of Arcane Hands (with the odd exception of the Bard). The damage absorbed is very low for a 5 th. level spell, so the item you are talking about would be pretty expensive if you use the spell to extrapolate from.
A last note (and a contradiction of my previous argument), it wouldn't hurt a groups class-balance more than a Wand of Cure Serious Wounds. It is possible to have healing in a group without resorting to Clerics, just remember to make it expensive and cumbersome compared to having a Cleric.