The same thing happened when i bought my punto.
The chances are that it has been sitting around on the forecourt or in a garage for a while before you bought it? It's probably just dust or some other random glitch that has affected one of the sensors.
Don't worry the airbag won't suddenly explode in your face!
I took it back to the dealer under warranty and stood and watched as the mechanic took a section of dashboard off to reveal the onboard computer, to which he attached a laptop. 5mins later he had reset the computer and the warning light doesn't come on any more.
Don't take it to a fiat dealer cause they'll charge you a fortune, but if you do, don't let them charge you more than say £20 for the job...
NOTHING NEEDS REPLACED... its just a computer reboot and 5-10mins of their mechanics time, for which they probably charge on an hourly basis.