I have been reading the FAQs at witopia.net, publicvpn.com, and other VPN providers, and I see why their services might be valuable when I use my computer or smartphone at a WiFi hotspot. But what about when I use my smartphone with AT&T's 3g cellular data? Will a VPN make my data more secure, or does 3g already include comparable encryption?
3G does include encryption, however, it has been broken and cannot be assumed to be secure.
If you are handling sensitive data on any wireless link, you should always encrypt. For many jobs the HTTPS encryption available in web browsers is sufficient. So if you wanted to read your personal emails via a web browser, that is all you need. Similarly, if using a mobile email client, ensuring that POP3S or IMAPS is enabled is sufficient.
If, on the other hand you need to log in to an enterprise system containing customer or payments data, you really should be using a VPN so that the end-to-end conversation is encrypted.
At the end of the day, even if 3G and 4G are totally secure, you have the internet at the other end of it, which is definitely not secure. So yes, VPN will make your cellular connections more secure.
Then again, a VPN just gets you securely onto a remote network. From there, if you browse the internet, then again, it's just going back out over the internet. But if your aim is to more securely get to a remote network, then yes, VPN is the way to go.
The other thing VPNs can provide in some cases is anonymity (assuming you don't then go and do something personally identifiable like log into your facebook account or MSN or whatever via the link!), and in this case, VPNs definitely improve on the situation vs vanilla 3G/4G cellular internet. But don't confuse anonymity with security :)