In a nutshell, ETOPS says how far away a twin-engine plane can be from an airport at any time (for emergency landing purposes). Historically, ETOPS limited twin-engine planes to being within 120 minutes flight time of an airport. More recently, due to experience and engineering improvements, ETOPS regulations now allow many twin-engine planes to be up to 240 minutes from an airport.
For the 787, Boeing is even targeting 330 minute ETOPS. Once they achieve that, there will essentially be no routes outside the range of a 787, leaving passenger capacity as the only reason to consider a super-jumbo. Given fuel costs, and improved route planning, there just isn't enough financial incentive to run a super-jumbo when a modern twin-engine wide-body with ETOPS240 or ETOPS330 will do.
Edit: As a side note, ETOPS improvements are also why all the good ol' 3-engine birds (727, L-1011, DC-10, MD-11) have been put to pasture.
That wikipedia page desperately needs a more readable introduction. Thanks for the info.
"The FAA has decided to use the single term, 'ExTended OPerationS,' or ETOPS, for all affected operations regardless of the number of engines on the airplane."
(my capitalisation)
That's also the reason why the 747-8F is the better-selling variant: cargo airlines love the 747 freighters (and from the beginning, the 747 was designed for this, in anticipation of them being retired out of passenger service and replaced by supersonic airliners that never really materialized), even if the passenger versions are no longer a good fit for the market.
But that doesn't mean it would be economic.
For various reasons - not least fulfilment delays because of manufacturing issues - the A380F hasn't been prototyped yet, and probably never will be.
.. except in cargo operations, where the MD-11F is all the rage. Cargo is exempt from ETOPS, which is why you see trijets still operating for FedEx, DHL, etc.
edit: I see ubernostrum said almost the exact same thing below! I'll leave the comment, as he/she didn't mention the ETOPS cargo exemption.