With the advent of the 4th edition of the rules of Dungeons & Dragons, Forgotten Realms is the first world to be updated to the new edition. Changes in edition generally mean changes in how some rules work, and those changes have sometimes been "explained" by big events in the Realms (often nicknamed RSE, for Realms-Shaking Events).
The best known RSE is most likely the Time of Troubles, when the gods walked the mortal lands, some gods were killed (e.g., Bhaal, god of murder, Myrkul, Bane - who later reappeared, even though he has not yet reappeared in FK -), and some mortals ascended to godhood (Cyric, Midnight/Mystra). The disppearance of the god of murder and assassins for example "explained" the fact that "assassin", which used to be a player class, got the axe. The change from Old Mystra to New Mystra/Midnight explained changes in how magic worked.
The move to 4th edition is also going to introduce many rule changes, and, once again, magic will be modified (moving away from the Vancian system [i.e., the memorization system] to a more flexible system, going from spells that can be cast once a day to spells that can be cast once per "encounter", and so on). The new RSE that Wizards of the Coast created to explain those changes is called the Spellplague (an event that (indirectly) caused the death of one archmage already, in the adventure described here - spoilers ahead!).
Now, as with many changes, there have been many fears, many rumours, many "They are killing the Realms!" comments, many "Nice change! That might make me want to play in the Realms!" answers, and so on. A freely readable chapter from a soon-to-be-published novel describes an event set 100 years in the future, where Drizzt muses over what happened to the Realms in the meantime, indicating that several good gods were killed, that many civilizations fell, and that the Realms became a much darker place (you can find the free chapter here). That frightened many fans of the Forgotten Realms, who saw in it the announcement that civilizations they grew to love (Cormyr, Sembia, the Sword Coast, ...) would disappear.
Another load of rumour came yesterday/today when the first fans got their hands on the soon-to-be-released product "A Grand History of the Realms" (see here), a - in my opinion very interesting - book that compiles a timeline of the Realms as well as several goodies (excerpts from "historical" texts, royal genealogical trees, ...). The timeline ends in 1385 DR with the following events (last warning: spoilers ahead!).
In 1384 DR, Tyr kills Helm in a romantic misunderstanding over the heart of Tymora. Though nothing can be proved, the gods believe that Cyric is somehow involved in Helm's death.
In 1385 DR, With help from Shar, Cyric murders Mystra in Dweomerheart, destroying the plane and Savras, as well as sending Azuth and Velsharoon into the Astral plane.
Magic bursts from the bonds of the Weave. Thousands of mages are driven insane or destroyed, and the very subsatnce of the world becomes mutable beneath the veils of azure fire that dance across the sky.
Cyric is imprisoned on his home plane for 1000 years by Tyr, Lathander, and Sune for his crimes. This event is what is known as the Spellplague