The pre-Cataclysm class changing patch has landed on the PTR and is alreadyavailable via the background downloader on the live servers. Official patch notes are below, but the highlights are:

• New launcher window that will stream patches allowing players to log in before patching is done
• Reforging: New NPCs in major towns will be able to transform 40% of a stat on your items into another stat that isn't already there
• Massive class, skills, talent and ability overhaul for all classes.
• New mastery system implemented
• Rage has been normalized
• Ammo is no longer in the game

Full notes from official page:

General

• Cataclysm Launcher Update
  • The new Cataclysm Launcher allows you to stream game data while you play. This will reduce the time you spend downloading, installing, and patching World of Warcraft. We'd like to get your feedback on the toolset and gameplay experience. Please head to the forums here for additional information.
• Reforging
  • Players can now visit special Reforging NPCs in capital cities located near Enchanting trainers. 40% of any one secondary stat on an item can be can be allocated to add a new secondary stat to the item. The stat being added cannot already exist on the item. An item cannot have more than one reforged stat at a time, though the Reforging of an item can be undone and changed.
• Copied Test Realm characters are not copied with their achievement history in order to better facilitate the character copy process.

Classes: General

• Many class talents, spells, and abilities have been overhauled, added, or removed.
• All spell and ability tooltips will now display damage averages rather than minimum and maximum damage potential. This feature is on by default but can be changed via the Display settings under User Interface options.
• Ammo has been removed from the game.
• The character levels at which classes gain new spells and abilities have changed.
• Many item and class stats have been changed or removed.
• Mastery rating has been added as a new stat which provides unique benefits for characters based on class and talent specialization. Mastery rating does not currently exist on items in the game, however, Reforging can be used to add Mastery rating as a secondary stat to existing items.
• Rage has been normalized.
• Spells and abilities no longer have multiple ranks and now scale with character level.
• Talent trees have been altered. All player talent specializations have been reset, allowing for free re-specialization.
  • Each specialization has been reduced to a 31-point talent tree.
  • Players will now get a total of 41 talent points to spend.
  • Players will be asked to choose a specialization at level 10. Doing so will result in the unlocking of a spell or ability unique to that specialization, as well as one or more passive bonuses.
  • Once a talent specialization has been chosen, players may only place points in the primary tree until at least 31 points have been spent there.
  • The initial announcement regarding these talent tree changes can be found on our Cataclysm forum.

Death Knights

• A dedicated tanking tree (Blood) has been implemented.
• The effects of Blood Presence and Frost Presence have been exchanged.
• The way in which runes recharge has changed.

Druids

• A new Eclipse mechanic has been added for druids.

Hunters

• Five pets can now be stored with the hunter at all times. Any one of these five pets can be summoned via the Call Pet ability.
• Focus has replaced mana as a new resource.
• Hunters now start with a pet at level 1.
• The Stable will now store 20 pets. If a pet is moved into the Stable, its talents are wiped.

Mages

• There is now a new spell fly-out UI feature for mage portals. Clicking on the Teleport or Portal buttons will expand the list of choices available.

Paladins

• Paladins now have a new resource bar.

Warlocks

• There is now a new spell fly-out UI feature for warlock pets. Clicking on this single button will open up the list of available pets to summon.
• Soul Shards have been removed from the game as items. Instead, they now exist as a resource system necessary for using, or altering the mechanics of certain spells.

Items

• Resilience no longer reduces the chance a player will be critically hit by an opponent.
• Durability: cloth, leather, and mail now have as many points of durability as plate, making the repair cost on death much closer to equal for all classes.

Professions

• The glyph system has been updated to now feature three different types of glyphs. Many class glyphs have been added, altered, or moved to different glyph types.

User Interface

• Spell Alerts have been added to notify players when procs on select spells and abilities occur. A visual notification will display around the character and the activated ability will be highlighted on the action bar.
• The Arena Teams pane has been improved.
• The Character pane has been redesigned. Character stats can now be shown or hidden via the Show All Details/Hide All Details button on the bottom left. When shown, all character stats will be visible in a window to the right of the pane. Stat sections can be reorganized by clicking and dragging them up or down the pane.
• The Professions panes now have more filtering and search functionality, as well as buttons to easily link recipes in the chat frame.
• A Professions screen has been added to the Spellbook to better display the details of a character's primary and secondary professions.
• Many of the yellow System Messages that would display in the chat frame when leveling up have been removed.
• New text animations now alert players as they level up, also providing information on when new talents, spells and abilities are available.
• Professions and Class Trainers windows have been altered to more clearly display available and upcoming purchases.
• New Raid frames are now. They can also be used in place of the standard 5-player party interface via the User Interface options.
• The Spellbook interface has been improved for greater ease-of-use and visual appeal.
• For additional notes on Lua and XML changes please visit the UI & Macros forum.