As a consumer, the last thing I want is more plastic in my wallet. Something like Cardstar (http://www.mycardstar.com/) would be a start, but maybe that's a typical early adopter HN attitude more than a typical consumer response.
Your landing page needs simpler, bigger, attention-grabbing text.
Also, I'll second the comment below about the big bright signup button. The CardStar link above has a nice example that you might draw inspiration from.
EDIT: Just watched a video from a link on HN:
http://www.avc.com/a_vc/2010/05/draw-your-ideas.html
And the website behind Jack Dorsey's new idea:
He makes the point in the video that 90% of America uses plastic to pay for stuff, but only 2% of America can accept payments from plastic sources.
If you can come up with a killer common-sense line like that (and a killer landing page like Square's), that will become the raison d'etre for your concept.