Hey, welcome to the forums! Great screen name!
what PB do you have? Any chance it's still covered by Applecare? What OS are you running? The fact that the cable works when propped up is not good news, seems to indicate a bad connection to the logic board.
If it's out of warranty, you can get ethernet PC-cards, which would be cheaper than getting it repaired.
But, out of curiosity, can you take it to another ethernet hub and check the connection with multiple cables at that hub? Also, if you're running OS X, and if your model starts up in OS 9, does ethernet work in OS 9?
Conversely, if you're running X, here's good troubleshooting tip: Establish another admin user account and startup in that account. If your problem disappears, then you've got corrupted prefs in the main user account. I don't think that will help you with this ethernet problem, but grasping at straws sometimes works?