From Sherdog:

A revised edition of the Unified Rules of Mixed Martial Arts was passed Thursday by the Association of Boxing Commissions via a majority vote at the ABC's annual conference.

The amended document -- drafted by the ABC's MMA Chairman Dale Kliparchuk, New Jersey State Athletic Control Board Deputy Attorney Nick Lembo, ABC President Timothy Leuckenhoff and veteran referee "Big" John McCarthy -- primarily provided clarifications to the already existing Unified Rules instituted in April 2001 that serve as a regulatory beacon for the sport throughout North America.

...

Forty regulatory bodies were in attendance for the annual event. The California State Athletic Commission and the Nevada State Athletic Commission, both known for regulating a large amount of MMA events in their states, did not attend.

Notable rules changes include:

• The back of the head is clearly defined as the "Mohawk" definition -- "crown of the head down the centerline of the skull into the spine, with a one-inch variance to each side."
• Downward elbow strikes are now legal unless aimed at protected areas.
• Smothering the mouth or nose of an opponent is now a foul.
• Weight classes are changed as well:

Fourteen weight classes are now defined in the men's division under the Unified Rules, beginning at the 105-pound flyweight division and moving up in 10-pound increments to super heavyweight at 265 pounds and over. In particular, the welterweight division will now range from 165-175 pounds, while light heavyweight fighters will be specified between 205-225 pounds. Heavyweight entrants will be asked to weigh in between 225-265 pounds.

UPDATE: via FightOpinion here's the complete breakdown of the new weight classes:

MEN’S DIVISIONS

Flyweight (Up to 105 pounds)
Super Flyweight (Over 105.1 to 115 lbs)
Bantamweight (Over 115.1 to 125 lbs)
Super Bantamweight (Over 125.1 to 135 lbs)
Featherweight (Over 135.1 to 145 lbs)
Lightweight (Over 145.1 to 155 lbs)
Super Lightweight (Over 155.1 to 165 lbs)
Welterweight (Over 165.1 to 175 lbs)
Super Welterweight (Over 175.1 to 185 lbs)
Middleweight (Over 185.1 to 195 lbs)
Super Middleweight (Over 195.1 to 205 lbs)
Light Heavyweight (Over 205.1 to 225 lbs)
Heavyweight (Over 225.1 to 265 lbs)
Super Heavyweight (Over 265.1 pounds)

WOMEN’S DIVISIONS

Flyweight (95 lbs. and below)
Bantamweight (95.1-105 lbs.)
Featherweight (105.1-115 lbs.)
Lightweight (115.1-125 lbs.)
Welterweight (125.1-135 lbs.)
Middleweight (135.1-145 lbs.)
Light Heavyweight (145.1-155 lbs.)
Cruiserweight (155.1-165 lbs.)
Heavyweight (165.1-185 lbs.)
Super Heavyweight (185.1 and above)

Download the full rules here.