According to Amazon (https://kdk.amazon.com/gp/vendor/sign-in?ie=UTF8&origina...), if you are offering active contents that have access to their Whispernet mobile network, unless you are selling your app on a monthly subscription basis, you are limited to use at most 100KB / month for a single user.
Yeah, it kind of makes it an awesome way to check in on your servers while you're on the go, with the free 3G on Kindles. Of course you'll get similar (100mb free 3g) with Chrome netbooks when they come out.
I applied for kdk access when it was first announced in January. Never a word. I think Amazon moved too slow here, because now I don't really care. I've moved on and enjoy building for the iPad.
Moreover, I cannot imagine Amazon is happy about 3rd-party code accessing the 3G which is paid by Amazon...