I2C is not for long-haul networking. The name is "inter-IC communication" - it's designed for chips on a board to talk to each other. If your cables aren't on the order of inches, you're asking for trouble.
Someone (Philips/NXI?) has an IC that can be used in pairs to extend it over longer distances.
Philips/NXI also make I2C bus muxes that can do voltage level translation. This is handy if you have multiple devices with the same address and have that pesky 5V/3.3V problem. The muxes are addressed over I2C, so there aren't even extra wires needed (although the code gets a bit more complicated).
-j