The Heavy Update is live in Team Fortress 2, but the patch notes haven't yet shown up on the Steam website, so here they are from Steam itself:

Updates to Team Fortress 2 have been released. The updates will be applied automatically when your Steam client is restarted. The specific changes include:

Added 3 Heavy unlockables
        • Natascha, The Sandvich, The Killing Gloves of Boxing
        • Added 35 new Heavy achievements

Added Arena game mode
        • 5 arenas: Lumberyard, Ravine, Well, Granary, Badlands

Added 2 new maps
        • Badwater Basin, a new payload map
        • cp_steel, a community map by Jamie "Fishbus" Manson

Changes
        • Added server map timeleft to upper right of scoreboard
        • Added new firing sounds to The Backburner
        • Removed the Custom Tab in the serverbrowser, and added the Tags field to the base Internet Tab
        • Increased The Kritzkrieg's uber-charge rate bonus from 10% to 25%
        • 32 player servers can now support an extra slot of SourceTV
        • Made the freezecam item panel description easier to read
        • Players now broadcast whether they chose a team directly or use autoteam
        • Added proper handling of cart blocking recognition to Payload maps

Bug fixes
        • Fixed bug where switching from flaregun-wielding Pyro to another class meant you didn't get a full ammo loadout on first spawn
        • Fixed Spy watch arm not using team skins
        • Fixed weapon switching preventing you from using secondary attacks for a 0.2s window after the primary became available
        • Fixed players getting stuck in a bad animation state when class switching while spinning the minigun
        • Fixed a loophole that allowed players to pickup weapons of other classes
        • Fixed a bug that resulted in players earning more crit chance bonus in large, single damage events than they are supposed to
        • Fixed an issue with the floating HUD +X health indicators that resulted in them appearing incorrectly
        • Fixed a bug which caused the minigun spinning & firing sounds to occasionally stop while the Heavy was still firing
        • Fixes the Loadout player model panel causing the in-game player facial animation to perform incorrectly
        • Fixed spectators not having the target ID labels display correctly
        • Fixed the cart alarm sound occasionally getting stuck on in Payload maps