If you have a device with WiFi AND 3G it works a little differently.
The @free address will let you send to the Kindle wirelessly but it will NEVER be delivered via 3G. So if you have no WiFi network available and only connect via 3G, the document will never show up wirelessly. You WILL get a note back to your regular email address that the 'converted' document is waiting for you to side-load. (That's basically what it says but the words are different.)
If you use the other address -- without 'free' in it -- and haven't set the charge limit to $0.00, you can send things and they will be delivered whether you are connected via Wifi or 3G. If they get delivered via 3G, you'll be charged 15¢ per MB or part of MB. If the charge limit is set at zero, that means you have not authorized them to charge you so they won't send it by 3G. It essentially 'flips' to the @free address and will work as explained above.
Thanks, Ann. So if this capability is true on the Touch with 3G wireless, for practical purposes you could receive a message from someone via wireless for 15cents anywhere your 3G wireless is connecting. So if you are traveling abroad for instance and don't have wifi, though you thus wouldn't have email access (ie, to yahoo or gmail), you could still get communication via your kindle address.
That wouldn't help with outgoing communication, but could save you an expensive incoming phone call here and there if that is true.
PS: I found this link which has been helpful, though I'm still absorbing it
http://www.amazon.com/gp/help/customer/display.html/?nodeId=200767340#sendingpdocs