OP could probably make it more explicit since it's not just a JSONP service, but also a CORS relay.
1) security
a) I won't trust you with my API keys
b) If you get hacked, ANY javascript can be executed on my pages
3) I can implement your service with a 3-line PHP script. Why would I use your service?
3 -- it is even simpler than that; the source can be found on github. At the very least, services such as this are useful as you are still developing.
For the viability of a commercial version of this project, I think the biggest question is: why would somebody use this instead of Yahoo's YQL?
I mean do you have something like a load balancer or proxy or something that will allow it to scale horizontally in case a few people really take advantage or it becomes popular?
It seems like leaving it totally free would probably end up with it just not being worthwhile to maintain.. but if there is a little money going in and at least one well-known service starts using it then I would totally consider using it.
In terms of scalability, the app is doing very little processing so it's really just a matter of how many concurrent connections Node can handle in & out, but I've read it's capable of hundreds of thousands or more. Let me know if you are seeing high latency.
While I think it is a cool service if a person is just hacking around, it is not something I would use with any revenue generating project.
It was to give a JSON API for http://bootswatch.com
It would be easy enough for anyone to host it on something like nodester.com or cloudfoundry.com and have it just for your own JSON data.
Currently you can accomplish the same thing using Yahoo's YQL, if you'd rather trust a big company to stay up. Here's an example: http://y.ahoo.it/b7rnO
I'm using it on my website to parse my project's history.json file: http://ruhoh.com/latest-updates/, check the source on that to see how it works.
Be warned that YQL transforms arrays containing one element into strings =/, terrible I know but it works.
ProxyPass /api/ http://destination.com/api/ ProxyPassReverse /api/ http://destination.com/api/