Does Global Thermonuclear War ring any bells? It was one of the hidden multiplayer modes in Modern Warfare 2 that Josh told us about just the other day. Having opened up the developer console, which, by default is blocked, players were able to gain access to otherwise unplayable modes.
GTW was one such mode, tasking players with "capturing" a nuke for 90 seconds to detonate it. Now, thanks to the persistent bunch at ModernWarfail2, everyone has the opportunity to witness the mayhem.
Since it's only a single guy showing this off, the build up is really tedious. But I'd be willing to bet a full match would be tense, frustrating, and most importantly fun. Whether or not this mode will A) get the paid DLC treatment, B) one day get unlocked for free, or C) never be officially released remains to be seen.
As a quick aside, the "getting a virus by way of IWNet" was a false alarm. No need to panic, for once.