Earlier quoted context omitted.
Sarcasm that's marked is not sarcasm.
idlewords or not, that’s not true. Verbal sarcasm is usually 'marked' by its delivery. (Can you hear Homer Simpson's sarcastic voice in your head?) In my experience, seeing an explicit (sarcasm) at the end of a post has never ruined the post for me. Certainly I spotted the sarcasm here myself, but clearly many didn’t.
The function of sarcasm almost seems to be a kind of social 'weeding'... "Do you think so badly of me that you don't realise that the horrible thing I just said is the opposite of my real view?" / "Are you too stupid to know when someone's being sarcastic?". The deliverer of the sarcasm gets confirmation that people know she isn't that much of a dick, the recipient gets to demonstrate that they 'get it'. Validation all around.
I think the cues are still there in physical interactions over here, they're just not as garish as a Homer Simpson sarcastic voice. "American-style sarcasm" exists as a phrase :) I realise the rise of internet/text communication will dilute these subtleties out of our lives but for now I'm one of the people who finds the /s tag very disappointing.