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Any reason you think people will spread trust instead of consolidating of abunch of instant transaction providers when time is critical?
Maybe you're right, but if you are, that's a huge reason not to implement this. We should encourage proliferation of instant providers otherwise we start becoming VISA all over again. That's a future for Bitcoin I'm not excited about: "Use one of these 4 companies, or you need to wait an impractical amount of time before your transaction will go through."
Come to think of it, is the payment protocol really the place to put this instant provider signature or should it be in the actual Bitcoin transaction? If we don't believe there is a valid practical solution to doublespends (some people have already emailed me critical feedback on my proposal) then we absolutely need a trust network, but we would also want it to be part of the public ledger for everyone to see.