What is the significance of the symbol in Tanaka's eyes? At the end of the Thanatos Report OVA (set after the events of the .hack//Sekai no Mukou ni movie) after Tanaka picks up a certain chip, a symbol appears in his eyes. 

What is the significance of this symbol? What does it mean for Tanaka?
That's a "doubleware" crest, meaning that aside from Tokio (from //Link and //Versus) Tanaka is the only other doubleware revealed up until now.

Doubleware humans can withstand Real Digitize themselves within the virtual world without worrying about "Outer Dependency Syndrome," the gradual erosion of their mind and digital body.