>Now where IS that Bill Buckley fellow when we need him?

I don't think Buckley is quixotic enough to take on the OED on *this matter; see for example:

"They were given physical, social and psychological examinations and then, placed in quarantine, cold viruses were deposited in their nasal passages."
-W.F. Buckley

the earliest citation that the OED has for the plural is this: 1908 Jrnl. Compar. Path. & Therapeutics XIII. 59 "Filters which are efficient for the arrest of the smallest of the known visible microbes allow the viruses of these diseases to pass through their pores."

as to the original question, why not virii?, here is an intersting discussion: http://language.perl.com/misc/virus.html