Justin and I did a test on some plain phone wire we had running between two buildings at work. There are two buildings, and there is 430 ft of plain jane generic (read: crap) phone wire. So we hooked up his laptop up to my laptop through this wire.
For those of you that don't know, the spec for Cat5 ethernet is 318ft, and that's over Cat5 which is much higher quality. Two wire phone wire it not twisted or shielded at all. We were able to do full speed transfer (10mbs) over that connection. I was expecting some small speed problems, or some packet loss. But there was NONE!