My opinion:
I think it's awesome that you are using json. :)
A standard that I can think of that involves webhooks is Pubsubhubbub.
Maybe there should be some type of wiki for a list of proposed
standards/formats.
Also, I think it's important to remember that these standards/formats
live on a different layer that webhooks.
And webhooks shouldn't be limited by the implementations/data formats.
-Drew LeSueur
[1]
http://code.google.com/p/pubsubhubbub/