Is data transmitted over 3G secure? When data is transferred from my iPhone via 3G, is it encrypted, or is it relatively simple for a hacker to read the data that's transferred to AT&amp;T's cell tower? What about after it reaches the tower?
3G can be secure or insecure, it's really down to the particular implementation.  If you are worried about your data while tethering or using a 3G iPad/iPhone then look at it this way, it's more secure than using free/unsecured WiFi hotspots/networks.

Really the answer to your question is that 3G is pretty secure, but it has its flaws.

3G is encrypted, the most common encryption algorithms have been cracked, with the right equipment somebody could intercept your information wirelessly.  However, they would need some knowledge, some money and some motivation to do it.  Then in addition to that, they would need your device to send unencrypted data (non https) so it could be deciphered.  All in all it's pretty unlikely but certainly possible that your information could be intercepted.  However, this is a risk for anyone transmitting data anywhere and is not isolated to 3G/WiFi or other mobile device communication methods.

Try a google search on 3G security and you'll find plenty of information on the flaws and security holes and how they might be exploited. 

Just as an example, here are a couple of presentations:

3G Security Overview  

blackhat.com powerpoint