According to Paul O'Brien of Modaco, there is a 3G version of Nexus 7 in production right now ad it will be coming out in around 6 weeks.
As we know, the Nexus 7 is a brilliant device (read Nexus 7 review) and is being sold at cost price by ASUS and Google. They want to get Android out there as much as possible and are doing so through this tablet which has been massively popular.
With it being sold at cost price some features were not included to keep the price down, such as a camera, MicroSD card slot and 3G connection. But it looks like a 3G version is now coming and it's no doubt because of how many people have been demanding it. I'm sure some wouldn't mind paying the extra to get a 3G version.
It's just a shame they didn't release a 3G version when the tablet was actually launched because I'm sure a lot of WiFi version owners are now going to be gutted.