What happens if I accidentally connect an Ethernet device to my 1-wire bus?
The voltages on Ethernet (ranging from -2.5V to +2.5V) are not compatible with the 5V the DS18B20 uses. The easiest way to prevent damage is to use different pins on the modular plug you're using for the sensors. Standard 10/100 Ethernet cabling (including POE, Power over Ethernet) uses pins 1, 2, 3 and 6. That leaves pins 4, 5, 7 and 8 free for hooking up your sensors.

If you can steer clear from cross-over cables and or gigabit capable equipment you should be fine for your project.

http://en.wikipedia.org/wiki/Ethernet_over_twisted_pair