Players will be required to exit their play sessions in order to download and install the update "shortly," said Bungie at 12:56 p.m. ET. The long-awaited update includes a host of weapon balance tweaks, the addition of matchmaking for weekly heroic strikes, some interface updates and a fix for the dastardly heavy ammo bug.
You can check out the full patch notes on Bungie's website. Also listed in the notes are some notable changes to the Crucible — for instance, enemies will no longer drop heavy ammo when killed, and they'll now drop much less special ammo — and networking improvements. Patch 1.1.1 also introduces a new bug affecting the Vex Mythoclast exotic fusion rifle; Bungie says it will fix that issue in a future update.