Points taken. My post was certainly not meant as a "very serious charge", and I doubt anyone took it that way. It was meant to be a bit of a barb, because I think it's slack just to dump a bunch of text in from an unnamed source and post it without attribution.

I do think the analogy of internet forum = casual conversation fails in this case. In a casual conversation, you can't suddenly start spouting word for word from an undocumented source unless you have some kind of freak photographic memory.

I stand by my larger point, which is you should include attribution when posting large chunks of material from outside sources, even if it's just starting your post with "From Wikipedia", and then putting quotes around the block of text.

There are many different views on plagiarism, though, and intelligent people can disagree.

And now, for you entertainment
Tom Lehrer's musical take on the topic of plagerism