Hmmm, wouldn't call it a waveguide, they are rectangular in shape, but it certainly looks like a highly directional receiving antenna of some sort (probably a yagi array). I suspect they have moved onto other people's networks as it's not pointing at your house anymore. With a directional receiver like this, they can probably pick up your wireless, even on its reduced power, but as you've now set the encryption to WPA2, they won't be able to crack it. They will have put this antenna up and moved it around until they managed to find somebody with no/WEP encryption on their wireless.
There's plenty of people out there who either don't bother with security on their networks at all, naively assuming that there's nothing to be worried about on a quiet residential estate, or think that as they have WEP security enabled that they are safe.
Even if the people who are logging onto your wireless network aren't using it for any malicious activity, they are stealing your bandwidth, stealing a service you are paying for. One of my work colleagues said to me while ago he'd doubled his internet speed for free
"How did you manage that?"
"I've logged onto next door's network, he's got it wide open. I can browse through all the files on his computer."
He later popped round to his neighbour and told him what he could do, giving him some suggestions on how he could improve his security.
Edit:
Here's how to make your own wireless hacking antenna
http://www.oreillynet.com/etel/blog/...p_er_chip.html
using a pringles tube, it looks like your neighbours have used a bit of drain pipe to weatherproof it.