My blue tang had it too, about a year and a half ago or so. I tried the metronidazole (Aquarium stocked it, might still). It would work to knock the ich down, but the ich would always come back (despite continuing to feed well after the spots went away, as cee suggested). But it was enough to keep the ich from getting completely out of control, and it was safe in my reef.
Some reading online suggested that a single infestation of c. irritans (ich) will only last something like (I think) 13 or so generations without the introduction of a different genetic line of its species. So I just kept using the metronidazole every time I saw a new round of spots, and within a few months they just stopped coming back. Now I haven't seen any sign of it since last summer, and the blue tang is happy and healthy.
It you go the "let it die out" method like I did, it's probably important not to introduce any new fish that haven't been fully quarrantined durng the waiting period, or you'd risk introducing a new genetic line of ich and re-starting your wait clock.
Jeff (vol_reefer)