Ok - trouble is over, but what a frustration!!!
I have a mythTV box in my livingroom and wanted a nice, hidden Cat5 cable connecting it to my router. How hard could that be? So, when my wife and I built our house, we asked for a line between my office and the livingroom. The idiot builders only placed the line and told me I would have to place the outlets myself. Jerks - sure, but I'll discuss that elsewhere...
I bought 2 LAN outlets and went about wiring them. I had trouble after trouble and eventually made my own tester using a breadboard I had laying around with an LED and 2 batteries. Everything was connecting properly but no go!
I eventually came across this Wikipedia article. I then re-wired each outlet EXACTLY like described in the diagram and it worked - at least so far with a different computer :)
I don't know what changed as before I had each wire directly going through (each outlet exactly the same) but by rearranging the wires to again be exactly the same, it now works.
I don't understand it, just glad I don't see myself doing it again!