I think you might be missing the point, all simulation continues while players are offline. Implementing consistent physics in an MMO is (very) hard, but having all players simulated even when they are logged off is something else entirely. The state of the art there is things like mail systems, auction houses, and skill queues.
I wouldn't be surprised if the offline simulation curtails your abilities somewhat, enabling a much lower CPU-effort simulation. We'll see, I guess :)
(There's also the point that a monthly subscription fee of $15 buys you a nice VPS slice these days. If you're willing to cut into the - significant - profit margin of MMOs, you have a lot of performance available for offline simulation. I'll stand by my judgment that it's a hard, but solvable, problem)
