Except for one fact. Some machines dissipate heat easier than others depending on their surface. I would not for instance put one of these hot machines on bed linen. A wooden or metal desk is a much better surface. A simple notebook stand can cost as little as $20 if you shop around. And the 120 degrees is not the max. The max is on http://www.apple.com/macbookair/specs.html are:
Line voltage: 100-240V AC
Frequency: 50Hz to 60Hz
Operating temperature: 50° to 95° F (10° to 35° C)
Storage temperature: -13° to 113° F (-24° to 45° C)
Relative humidity: 0% to 90% noncondensing
Maximum operating altitude: 10,000 feet
Maximum storage altitude: 15,000 feet
Maximum shipping altitude: 35,000 feet
Airplane cabin pressure typically is normal for 9000 feet, which is why it is safe to use in flight. Temperatures above are ambient external to computer temperatures.
SMC FanControl has recently been found to be 10.7 incompatible and cause more issues for computers that have been updated or new machines than using without that software. If you are experiencing other overheating symptoms such as unexpected kernel panics, or un-force quitable applications (command-option-escape is the command to force an application to quit via the Force Quit menu), or unprogrammed shut downs (either by Energy Saver or power button, or Apple menu -> Shut Down) only then would I worry about overheating. Now if your ambient temperature humidity is normal, and the fan is just noisy, get a pair of headphones to block the noise.
This is a user to user forum, and all we can do is suggest remedial actions to take. The fact you had 4 replacements and it didn't perform as expected says either the environment is hostile for it, or your expectations are excessive.
