I've had one of those 1TB Western Digital MyBook World drives for about a year now. Paid about $180 US for it. It's pretty slow. But what I like about it is that it has a server built in (Linux-based, but that's totally transparent to the user). So if you hook it to a router via the buillt-in ethernet port, as long as the router is connected to the net, you can access it remotely from anywhere in the world.
When I'm out shooting on vacation or something, I load up my pics on my laptop daily, and then have them transfer home to the mothership for safe keeping. Assuming I have net access wherever I am, if the laptop gets stolen, or its drive fails, I always have a backup at home. Pretty slick.
Ideally, I'd have another 1TB drive on my network that just syncs up wiith the MyBook every night. That way, I'd have another backup if the MyBook failed, and it would not be visible outside my internal LAN. After the holidays, 1TB drives should be available for very little, so I'll probably buy one then.