3G (and 4G which is coming out now) is simply a cell network upgrade allowing faster data transfer... think of it like 108.11b/g/n formats for WiFi; range and speed increase as the technology gets better.
As for the OS, it's just a simple OS designed specifically for the iPhone hardware; taking advantages of its strengths to provide a good interface. They don't have to make it widely compatible to other hardware formats, so they don't have to make as many compromises. Could there be a virus to infect it... potentially, but it's not particularly likely.
There are 3 times as many mobile devices than there are pc's. Many contries are way ahead of the USA in embracing mobile devices into their every day life style.