The biggest concern, particularly down the road, is assuring people you're above board on security. When I mentioned it on G+, the first suggestion was that you guys could inject ads and such, which obviously would be a huge problem.

But generally if there's a solid paid model, a company can be pretty free and clear of those problems.

The other thing you would want to consider is Firefox and Chrome plugins where you could set specific sites to open via your protocol. Maybe Engadget doesn't work well on my computer for whatever reason, so I tell the extension to open all Engadget URLs through your service, or something like that.