This page goes into great detail about why ejection seats for airliners are a very ungood idea, so we'll just sit back and let them do the talking:
Pro: Safe recovery of passengers in the event of a catastrophic disaster.
Danger to maintainance crews
Danger to passengers due to accidental discharge
Possible injuries to passengers due to use
Larger seat area requires fewer seats in given area
Egress hatch requirement requires considerable redesign of cabin fuselage
Cost- Ejectable seat would add costs for:
a. Redesign costs
b. Development costs
c. Seats (military seats can cost upward of $100,000 per unit)
d. Periodic maintainance
e. Replacement parts
f. Maintainance crew training/explosive certification
Minimal time of use
...Parachutes require more than just a simple lap belt. At a minimum, the harness requirements include a pair of leg straps, a pair of shoulder straps and a chest strap. These straps must be adjusted for each individual to be a snug (read uncomfortable) fit for each passenger... In the case of a situation requiring a mass ejection, this would have to be delayed until ALL passengers AND crew are strapped in securely prior to depressurizing the cabin, blowing the hatches and initiating ejection.
When the cabin is depressurized and the hatches are jettisoned, the passengers would be exposed to the lower oxygen pressure in the upper atmostphere, the wind blast which would cause flail injuries and injuries by loose flying objects such as handbags, cameras, camcorders, trays, carry-on bags, and other objects.
Jettisoning a large number of hatches in the roof of an airliner will also cause significant changes in the aerodynamics of the aircraft leading to control problems for the flight crew.
The aircraft structure would require massive modification to make ejection seats feasable, including strengthing the cabin floor for the additional weight and the recoil of the seat firings. The cabin roof would have to be configured with the aforementioned hatches. The fuselage would therefore need a major increase in supports to allow it to hold its shape when the hatches were jettisoned. Overhead baggage compartments, and underseat storage would have to be eliminated to give adequite clearance above and, because of the seat depth, below. Legroom would have to be adjusted to make sure that adequite clearance was maintained on ejection to prevent leg injuries.
Mechanical ejection (spring/bungie) would provide inadequite thrust to ensure that passengers would clear the empennage. Compressed gas systems that would have enough force would provide too great of an initial force for safety. This means that pyrotechnic rocket/catapult systems be used. These systems would necessitate significantly increased training for maintainance personel, cabin and flight crews. The pyrotechnics would require maintainance on a regular basis in an explosive rated hanger with explosive rated storage.
The ejection sequence would have to be from the rear to the front of the cabin, with the flight crew being the last of all to be ejected. In an airliner with 30 rows of seats, the seats would have to be jettisoned in row sets, with seperation rockets to insure dispersal of the seats and prevent mid-air seat collisions. There would have to be a delay between rows for the same reason. This delay in military jets is in the vicinity of .4 to .5 second. This adds up to some 15-16 seconds for a full ejection of the aircraft. Seats that are unoccupied must be weighted to ensure that they seperate in a predictable path. While the seats are firing, the aircraft would be exposed to forces from the catapult charges, and the center of gravity would be changing rapidly. This would cause significant difficulty to the flight control system to maintain stability.
Passenger survival by seperating the passengers from the aircraft prior to impact is not an option that could be easily or cost effectivly implemented.