Here are some various airlines' policies on golf club bags.

DELTA
http://www.delta.com/traveling_checkin/baggage/special_baggage/fragile_bulky/sporting_goods/index.jsp#6

Golfing Equipment
One item of golfing equipment is accepted as part of a customer's free baggage allowance. One item of golfing equipment is defined as one or up to all of a combination of the following:

One golf bag containing not more than
14 golf clubs
12 golf balls
one pair of golf shoes

Items in excess of Delta's free bag allowance will be assessed an excess baggage charge.

Customers wishing to check golf clubs have three options:
1. Hard shell golf travel containers - no limited liability release required
2. All other golf travel bags (i.e., soft-sided bags) - limited liability release required

The limited liability release absolves Delta of responsibility for pre-existing damage to the clubs, or damage from the normal course of handling.

UNITED
http://www.united.com/page/article/0,1360,1032,00.html

Golfing Equipment (1 golf bag with clubs and 1 pair of shoes) must be enclosed in full-length protective cover. (vinyl/cloth/nylon/or hard sided case).

Carry-on bags
For travel worldwide on United Airlines, you may carry on one bag and one personal item such as a purse, briefcase, or laptop computer. A carry-on bag must fit under your seat or in the overhead bin and cannot weigh more than 50 lbs. Its linear dimensions (length + width + height) cannot exceed 9" x 14" x 22". United Airlines may require that a carry-on item travel as checked baggage if the item cannot be safely stowed on a particular flight.

United exempts the following personal items from the one piece limit:

Child safety seats for ticketed children
Assistive devices (e.g. canes, crutches, etc.)
Outer garments (e.g. coats, hats, etc.)
Small purses (no larger than 25 linear inches)
Briefcases
Laptop computers

Checked bags
For travel within the U.S. and Canada, and to many other destinations, you may travel with two free checked bags.

Your largest checked bag must have linear dimensions that do not exceed 62 inches and must weigh no more than 70 pounds.

Each checked bag must have the name of the person traveling on the outside of the bag.

You may substitute golfing equipment for one piece of the two allowable checked pieces. Military personnel may substitute one duffel/B-4 bag free in lieu of their largest checked piece. Contact United or your travel agent prior to your trip for details regarding acceptance and restrictions for these or other checked items.

If your ticketed itinerary includes any ground transportation, you must retrieve your luggage and transfer it between the airline and the ground transport.

If you are traveling outside of North America, United's free baggage allowance may vary. Contact United or your travel agent prior to your trip for details.

AMERICAN
http://www.aa.com/content/travelInformation/baggage/baggageAllowance.jhtml

Golf equipment must consist of no more than one golf bag with golf clubs, golf balls, and shoes.

CONTINENTAL
http://www.continental.com/travel/policies/baggage/default.asp

Golf Equipment
Continental accepts one item of golf equipment per customer in lieu of one piece of free baggage.

An item of golf equipment consists of:

1 golf bag containing not more than 14 golf clubs
12 golf balls
1 pair golf shoes.

All items must be properly encased in a suitable container. The golf bag must be covered or enclosed in a heavy, rigid carrying case. Continental shall not be liable for golf equipment presented without a carrying case.

Golf equipment carried in addition to the free baggage allowance will be assessed at the current excess baggage charge.

SOUTHWEST
http://www.southwest.com/travel_center/sports_equipment.html

Golf clubs that are transported in a hard, plastic travel case or a standard club bag are accepted as checked baggage. Golf clubs transported without a hood to cover the clubs, or clubs that are covered with a plastic bag, will be conditionally accepted. Golf equipment includes golf clubs, golf balls, and golf shoes, contained in one bag. Golf clubs are not allowed onboard as carryon items.

Free Baggage Allowance
Upon presentation by a fare-paying passenger of a valid ticket, Carrier will allow the following baggage to be transported without charge:

Checked Baggage - Three (3) pieces of baggage, each piece of which having outside measurements (i.e., the sum of the greatest outside length plus the greatest outside width plus the greatest outside height) not exceeding sixty-two (62) inches, not weighing more than seventy (70) pounds per piece, and provided such baggage is checked for carriage in the cargo hold of the aircraft.

Exception: Military passengers traveling on permanent change of station (PCS) orders will be permitted to substitute as checked baggage one (1) duffel bag, sea bag, or B-4 bag that exceeds the sixty-two (62) inch dimension, provided that same does not exceed seventy (70) pounds in weight and eighty (80) inches in size (length plus height plus width).