This is a tough subject!
One could say that imitation is the highest form of flattery, but that doesn't put bread on the original authors table.
The very least one should do is give credit back to the original author.
Like #denmarkguy said, once you post something on the web it is pretty much up for grabs.
But, does that mean that if I retweet something on twitter am I in violation of copyright?
I tend to agree with #traverse on this.
Take a snippet that you need to make your point and give a credit to the original author.
Thoughts are free, however, when they become words are they still free?
There is something called synchronicity where two or more people have come up with the same idea in different parts of the world at the same time (think laser and telephone) and only one gets credit. In the case of Alexander Bell he beat the other guy to the patent office by a very small amount of time and no one remembers the other guys name and Bell gets the credit.
As my favorite poet, Robert Service, once wrote in "The Shooting of Dan McGrew"
"I'm not so wise as the lawyer guys, but strictly between us two—
The woman that kissed him and—pinched his poke—was the lady that's known as Lou."