Nope, no shared wall, there's a bathroom in between the two rooms. Air ducts are also out of the question.

Well, that sux... I've never tried the powerline ethernet - but if it works anything like the powerline phone line then I wouldn't recommend it. Another option would be to get a couple of wireless PCI cards. You could get a couple for even cheaper than your powerline ethernet solution. There are a bunch listed here (although, the cards with an actual wireless card in them and not just a chassis start at about $38). You could then set up a direct link between your linux box and the other pc. I would trust that connection a lot more than one over the apartment power lines.

- trs