I've not done it myself, but I know a bloke who has a 4-port wireless modem router with two of the ports running in (long) Cat5 cables to wireless access points at the two ends of his property. That gives him coverage over a pretty extensive area.
The secret was in using high-quality (long range) wireless kit. It wasn't horribly expensive, either.
I've not tried those mains-borne devices but I've heard good reports about those, too. I don't know if you can hook a wireless access point onto one, though.