@Tomsem
The restriction may not be at the ISP level, it could very well be that the Kindle web browser itself is restricting http traffic to port 80 while on 3G. I did some quick fiddling and was able to get my NAS webserver to work for direct kindle downloads, talk about a media vault now I've got music, videos, and books at my disposal
Configuring routers for port forwarding isn't all that complicated, there are tons of examples on the web as well if you run into problems. I recommend trying something like this: Setup Calibre to run on say port 5000, then in your router forward port 5000 to port 5000 of whatever internal IP your calibre pc is using. While on 3G if you can point your kindle to
http://youipaddress:5000 and it works then my issue is definitely with my 3G provider. If it doesn't work then try again on wifi. This will answer a lot of questions for the rest of us. I can confirm at least where I am (Saskatchewan Canada) that I can only access port 80 over 3G (I'll try on wifi when I get home).