actually there is, in the wotlk beta, the 5% increase of corruption damage was bugged and stacked every time you refreshed it with haunt or drain life, and it reached some number on those unkillable mobs that was *something* like the gold cap is (gold cap is 2^32 i think...the damage cap was some number like that, too)there is no theoretical upper limit on ignite damage.
so yes, there is a theoretical damage limit.
PS: looks like the youtube video was deleted : (