Yup, I think this is something a lot of us have probably suffered, whether internet related or in the outer world.
I had some right classic cases from when I used to work for a company a while back and i'd be constantly throwing idea's into the pot and planning things and designing new stuff, yet almost everytime, this other pillock kept claiming all the credit for himself, trying to get browny points, it kind of backfired in the end though, cause as soon as I left, he suddenly didn't have any idea's at all and soon got into a lot of trouble for it.
I've had it endless times on the net though, where you help someone out with setting their site up because they haven't got a clue where to begin with setting up something simple like vBulletin, the next thing you know, they start spamming your users and emailing them asking them to join their forum as it's better than yours and funnily enough, you look at their forum sections, smilies, half their images and in cases their content and you don't have to think very hard where it came from...
Some people seem to think that everything is theirs and that the months and years of hard work you put into your work should be theirs by rights when ever they see fit to take it and use it themselves. Rather than putting in their own work, making their own site and having their own mark on the net.
For 3yrs i'd managed to get away with not having to use anti-leeching tools on my sites, but recently I suddenly saw my bandwidth useage shoot up by about 300% for no reason, so on checking, I found that someone had pretty much used my entire site as a clone, not even bothering to save any of my images to their own server... Mind you, I sharp put a stop to that one and I found the results hilarious, with the nice effect of 800x600 bandwidth thief images on the guilty site.