I gave an example of JSON some time ago that turns out to be wrong. I used single-quotes in my JSON example. I'll leave them there since the example works, but I just can't call it JSON.
If you're doing JSON-style coding by hand for your Ajax communications, you can get away with using any object literals. So why care if it fits the JSON standard? Well, there are parsers and tools out there which can work with the JSON format, and if you don't fit the standard, they might not work.
Moral of the story: it probably doesn't matter if you use the JSON standard until you have to interoperate with JSON tools or feel the need to offer your JSON data to the public. You should probably stop calling it JSON if you use single-quotes, though.