It is very unlikely that Canon will dump the EF-S mount any time soon. As you note it was derived from the EF mount, which is significantly older but still going strong. In fact, Canon's introduction of the EF-M derivative last year, if anything, indicated Canon is nowhere near phasing out the system. Camera mounts are not replaced nearly as often as other sorts of "technology" in part because photographers put so much money into a given system (look at Canon's L lens prices and you'll see why people are unwilling to frequently move to new systems). Not only is it prohibitive in pricing for people to move around, every time a company breaks system compatibility, it provides a great opportunity for photographers to check out the competition, something no major player wants to risk. (You aren't going to be lured by a single cool Olympus body if you have thousands of dollars sunk in Canon gear.)
While the EOS/EF system has been around since 1987, that actually makes it a young system comparatively. Nikon's F mount goes back to the late 1950's, Pentax's K mount goes to the 1970's and Minolta's A mount (aka Sony α mount) dates to the earlier part of the 80's. While other players have introduced new mounts, the ones that did were smaller, less significant players which either did not have older mounts or had small enough existing user bases that the risks of introducing a new system were outweighed with the advantages of a blank slate.
Canon's EF mount actually is aging quite well. At the time, Canon controversially killed its old FD mount, enabling it to move to a pure electronic mount system -- no mechanical link between the body and the lens -- for autofocus, whereas Nikon went through a long transition period wherein it used a mechanical shaft to achieve AF on its F mount lenses. Minolta (now Sony) and Pentax followed the same path as Nikon as far as a mechanical AF system, but in all cases, they've slowly moved to the sort of arrangement Canon used, retrofitting it on their old mounts. This leads to long term compatibility issues where some bodies can't use AF with some AF lenses, but avoided the one time pain Canon caused by starting a new system.
Why does all of this matter? Well, by Canon making that painful switch in the 80's, it set itself up for a very long life for the EF system. There isn't any need for a new mount. All of the sorts of electronics that become dated are either in the body or in a given lens, not the mount. The EF-S and EF-M subsets take care of the physical changes/opportunities that have occurred since the 1980's film era. The EF-S can offer advantages for crop body cameras like the Rebel series and xxD series, while the EF-M leverages the physical characteristics of compact non-reflex bodies to make smaller lenses. But the core electronic communication system remains the same, thus this isn't a progression towards a new system. You can mount a normal EF lens on an EOS M (EF-M mount) using an adapter and all the electronics work, because the contacts are the same.
In an nutshell: don't worry. No one can promise Canon won't do something crazy, but it is unlikely. If you have or plan to have a full frame body at some point (like the EOS 1D series, 5D series and 6D series), buy EF lenses. They'll work on any Canon body and thus are the most flexible. If you only plan on using crop bodies (like the Rebel, 60/70D and 7D), expect the EF-S mount to remain also for the long term, since they fit the vast bulk of the bodies Canon sells. Finally, if you want something tiny (right now, only the EOS M body), buying into EF-M lenses makes sense, but thanks to the official adapter, EF and EF-S lenses are perfectly safe bets too.