The FCC specifically addresses emergency communications as one of the important purposes of ham radio. It is nothing to be taken lightly, so don't ever joke around on the air about a serious situation. Remember "the boy who cried wolf"?
Hams are limited to operating only within the amateur radio service bands set-up by the FCC. The only time that you can deviate from this protocol is when you have a legitimate emergency. This would certainly lead into the category of life-and-death scenarios. Normal operating procedures restrict license holders from transmitting on frequencies outside of their license privileges unless there is a serious emergency in which you or someone needs assistance.
You've heard the terms "Mayday" (in voice mode) and "SOS" (for CW) used on television. These are terms that are actually used by radio operators when they have an emergency. The words SOS or Mayday should only be used in a life or property-threatening situation. If you transmit these words when under false pretenses - it is considered to be a "false or deceptive signal" - which is grounds for revoking a license. If you need to break into a conversation on a local repeater during an emergency, it is acceptable to say the word "Break" two or three times. Otherwise, if you just want to join a conversation, wait for a pause and say you callsign to get recognized.
During a disaster, emergency communications take priority over all other traffic, and only communications essential to providing relief to an area should be transmitted. Traffic concerning the immediate safety of human life is officially termed emergency traffic, while anything relating to a person's well-being is considered to be health and welfare traffic. Tactical callsigns, which are military-like names for locations and personnel, can be used during an emergency.
The FCC has the power to declare a temporary state of communication emergency. If this happens, the FCC will state what special conditions apply to communicating during that time period.
Field Day is an ARRL-sponsored emergency preparation exercise conducted each June that covers all of the country. The goal of the exercise is to get with hams in your area, set up a functional high-power station, and be the first to make contact with another station. Even though this is an emergency preparedness exercise, all normal operating rules apply - so that means that you can forget excusing yourself to break in on FM broadcast.
RACES, the Radio Amateur Civil Emergency Service, is a FCC-regulated "public service that provides a reserve communications group within government agencies in times of extraordinary need". Your local area probably has a chapter that you don't even know about. Civil defense organizations periodically hold RACES drills that can last up to one hour per week; you must be registered the responsible civil defense organization to participate. Click here to go to 47 CFR Part 97 concerning RACES to find out more.
Also, visit the RACES homepage.