Just wondering if 3G networks can be sniffed like public wifi networks, via wireshark for example (I'm almost sure this is not possible due to encryption, but I just want to make sure?)
Sure - any network can be sniffed, as long as the sniffer can be connected in the right place.
Communications between the phone and the cell antenna are encypted, though, so without some specialized equipment and processes, data is relatively safe on that path. After getting decrypted at the cell antenna site, though, it could be sniffed at one of hundreds of places inside the telco's network.
As in most things, define your threat model. Most people out wardriving are not going to have the equipment to decode 3G data into packets. But, if you're sending information worth $1 billion dollars over 3G, someone would easily spend $5 million dollars to be able to do it. Nothing is secure, and as long as you define your threat and have "sufficient" security for that, then you're OK.