There are a lot of enterprise VPN products around designed to make remote access more secure for mobile workers of all varieties.
As more of them use iPhones or iPads as their personal computing platforms (what's the smallest thing you can use and still call it a 'platform?') and sometimes their wallets, encryption will become more important.
Cell networking protocols are more complicated than plain-vanilla IP, but once you make the connection on most 3G or 4G networks (with permission from your carrier), you can send pretty much anything over them that you would over a regular IP network.
And regular IP, when broadcast in the clear, is not the most secure thing in the world.
One of the newer attempts to fix that comes from Kryptos Communications, which launched an iPhone app in December that uses 256-bit AES encryption and a 2048-bit key to keep conversations and data secure.
It's not a regular call, though. It makes peer-to-peer connections between your phone and that of the person you call, who also has to have a Kryptos client running.
Currently it runs only on Apple's iOS, but versions for Android and RIM Blackberries are in the works; none are famously secure.
The app, available from the iTunes App Store for $4.99 plus a service fee of $4.99 per month, creates a Secure voice over IP (SVoIP) connection, the secure protocol much of the Department of Defense uses for its communications.