Pricing in the ECAD space is weird (you're probably aware) because it goes 'free' -> $1K -> $10K. with very few intermediate points. What is more, the number of people who have been burned by having something 'in the cloud' from a smallish supplier is growing. So having private designs that would be 'gone' if your servers were offline would be an issue. And lastly the biggest thing most people pay for these days is simulation anyway, and that is something easily charged for. You might consider 'free schematics' private or public, but $20 to generate a PCB from one. You could work out a scheme where if I publish a schematic using upverter and someone comes along they can 'buy' a PCB for it and you split the revenue share with the schematic creator. You can work a partnership with one of the PCB houses so that it 'just works' or you can send them a zip file of the various bits (gerbers, drill file, tool file, etc).
Something you might offer for 'free' would be something like Google's ChartServer API but 'schematic server' API. That is something I've been building in my spare time off and on. You are a lot closer than I am and I've yet to figure out if I want to use HTTP POST or a split URL syntax.
Can you imagine something like:
<img src="http://upverter.com?sch=dropbox.com/chuck/schematics/foo.sch" height=200 width=300>