As others have pointed out, this sensor uses the so called one-wire bus (which is actually at least two wires and preferably three, but that's marketing for you), not I2C. There is a one-wire library available for Raspi, and it is even included in Adafruit's own Occidentalis distro. And yes, a one-wire bus can be extended for quite a distance.
Note though that Adafruit warns against using this sensor at 100°C or over, as the insulation may be damaged. Of course, you want
to keep the temperature inside a boiler below 100°C, which brings me to the point I wanted to make: For the love of $deity, if you are messing with boilers, make sure that the safeties are in place.
This means at least
one overtemperature thermostat or thermofuse, connected in series with the heating element, and
a pressure relief valve. Note that local regulations may be even stricter, but this is the minimum
common sense safety equipment for a boiler. If you want to know what can happen when the safeties are disabled, watch this: http://www.youtube.com/watch?v=hWnL8SipXT8