Of course, you'd only be able to use one at a time without an adapter of some nature, and I have no idea if the voltage over the center pair might damage your NIC or computer.

you definitely want to be careful here. even if your NIC is wired to ignore those pins that it doesn't use, the -48VDC ring voltage on an analog telephone line will quickly fry a NIC if wires get inadvertantly shorted or crossed. beware.

--dan.