"Valve said they were going to release a Left 4 Dead patch this week. Valve aren't in the habit of lying to us. The update just went live, and brings a number of improvements.

A ton of glitches and bugs have been fixed, along with a number of balance tweaks and gameplay changes (though sadly no extra versus maps). Best of these by a desolate, zombie-infested country mile is the one where it says "Increased chance of getting the Tank or Witch".

The update is live now for PC users, while for 360 owners, it'll turn up in the "coming weeks".

The full list of fixes and changes are below. Have at it.

General
- Fixed Survivors being able to climb surfaces marked for versus infected only.
- Fixed a class of SurvivorBot bugs dealing with rescuing downed players.
- Players can no longer grab ladders while flying through the air after a Tank punch.
- Shooting near a car with an alarm that has already fired the alarm will no longer make chirping noises.
- Improved loading time.
- Fixed mini-gun physics exploit.
- Fixed propane tanks (and other physics objects) causing players to fall through elevators.
- Fixed rare achievement bug issues.
- Fixed several map exploits.
- Fixed various match making issues.
- Fixed NAT traversal issues.

Versus Changes
- Added HUD elements to show status of other infected players.
- Changed color of infected player name in chat to red.
- Fixed exploit where players could spawn infected bots
- Fixed instance where a Survivor changing to the infected team would be attacked by infected bots.
- Normalized special infected melee damage.
- Made the following client commands cheat protected: "Kill" and "explode".
- Fixed exploit where infected players could run away and teleport back to gain health.
- Players can only change teams once per map.
- Players can't change teams while other players are still loading.
- Tank spawns at the same % through the map for both teams in versus mode.
- Made the Tank and Witch spawn directly on the escape route.
- Increased chance of getting the Tank or Witch.
- Fixed team swap issue.

Hunter
- Easier to pounce a Survivor who is meleeing.
- Increased Minimum damage a Hunter pounce does.

Smoker
- Fixed Smoker tongue tolerance.
- Smoker now has to be killed or the tongue destroyed for the tongue to break.
- Survivor's cannot bash someone off the tongue until the Survivor being pulled is paralyzed or hanging
- Tongue attacks that fail to paralyze or hang a Survivor will use the shorter ability delay timer.
- Fixed cases where the ability timer was not using the correct time.
- Fixed case where you could point at a Survivor but not register a tongue hit
- Fixed Smoker tongue not targeting and landing properly through PZ ghosts.
- Smoker tongue does damage every second while dragging paralyzed Survivors.

Tank
- Bashable objects now appear with a red glow.
- Tanks hitting a car with an alarm disables the alarm permanently.
- Tank frustration timer is only reset by hitting Survivors with rocks or fists.
- Reduced autoshotgun damage against Tanks.

Witch
- Witch spawns at the same % through the map for both teams.
- Avoids spawning within a certain % of the tank.
- Fixed an exploit where the Witch could be woken up and tricked into attacking Survivors."

Source