You aren't the first company to try grid computing on personal computers, so once people stop talking about how cool the idea of grid computing is or how to protect the web surfers rights, they'll start asking the hard questions. Like, how you're going to make money.
How do your paying customers feel about their data being strewn about the Internet? How does your processing interfere with the game that the user is playing? What kind of problems have you earmarked as being particularly parallelizable like this (ie what is your marketing plan?), and how usable is your API?
At the end of the day, is going through the effort to rewrite a program to work on distributed Java applets going to produce substantially more processing for the customer dollar than a $150 headless dual-core 1gz board with a gig of RAM running C++? You say that you can provide a $1500/yr savings over EC2, but this is only for applications that are "embarrassingly parallel", and will certainly require hiring a programmer to write code to run in tiny chunks on lots of clients. Will this cost less than $1,500? Worse, it will be money up front instead of spread out over a year.
You could say that the system is best leveraged by clients looking for a lot of compute power on a budget, but those are the people most likely to build their own grid. When you can get linux running on a dual-core 2.0gz for $200 / node, spending that money every year on an untested platform that will require substantial development work and vendor tie in, your service is going to be a tough sell.
You will be the only people interested in writing code for this platform, so please repost when you know which algorithm you can run to gross $10 million a year.