I think Slipshine does this--offers previews of their numerous comics, but charge for full access. For a single comic, though? No, I can't imagine a single webcomic good enough for me to pay for it. I've got enough absolutely wonderful webcomics to read that don't demand my money first.
Another thing is, Slipshine offers something that you can't easily find on the free side of the internet-- quality adult comics. There are a few, but they're rare compared to the non-adult comics. They're also already targeting an audience that is probably used to paying for this type of content.
Though I agree that if you gathered a bunch of great quality comics and offered them all in a way similar to how Slipshine does, you might have a higher chance of success.
Also wanted to point out that there was a lengthy post about micropayments over at www.webcomics.com this morning.