This may be common knowledge to everyone but me. In our new house I made two long cat-5 cables to span from a switch to the 2nd xmbc xbox and a PC in a remote bedroom. Well neither (longish) cable would work on either xbox, but they did work on my newest PC which has a built in TBase1000 chipset.
Today while recovering from a few mild Lariam days I am fixing a few loose ends at home. One is those pesky cat-5 cables. After following this image found on wikipedia the two cables now work perfectly:
It would appear that the order of the wires is very important. Of course I learned about twisted pairs in high school, and have taken three technical degrees. I just assumed that like colors go together in pairs, but it is not the case for Cat-5 at least. Apparently for TBase100 only two pairs are used and I guess the way I wired them signals were interfering at the ~100MHz transmission rates.
Hopefully this helps someone out there for the same frustration I had - are maybe everyone else reads instructions? It is also worth pointing out that Cat-5e is fine for TBase1000 - it works in my other house using el-cheapo Home Depot Cat5e Riser cable and it is a documented IEEE standard: http://en.wikipedia.org/wiki/Category_5_cable