Don.. on the iPhone (and other Smart Phones) there are 3 modes.
1. Cell phone to make calls
2. Data - to get email, internet and any auto update Apps (like stock listings, etc).
3. WiFi - same as Data but through WiFi
I am sure you knew the above but broke it down anyway.
When in CZM you don't get any roaming charges on #1 above if you don't make a call.
When in CZM you don't get any roaming charges on #3 above because WiFi is free at a WiFi hotspots or hotel you are staying.
#2 above. When in CZM if you have any Apps on your phone that auto update (I think Verizon calls it Push), like stock market quotes, GPS, mapping, email and all other programs that normally work on the Verizon 3G network while you are moving around and update automatically (a lot of programs do this).. this is where the Data Roaming charges will cost you a ton because they roam and update 24/7 as long as your phone is on.
Example.. Like GPS, if you walk from your hotel room to the restaurant, data Pushes info to your phone so as soon as you look at it the info is there, already pushed (updated) to your phone. Go back to your room and it tracks you. This is data roaming and costs nothing in the USA but a ton if outside the USA cause it runs 24/7.
This is what you have to turn off using my first post. The phone (for calling) and WiFi can remain on all the time and no charges apply unless you make a call.
But.. if you are in a WiFi hotspot and logged into a WiFi network all apps will update through WiFi but will quit updating as soon as you are out of the range of the hotspot. WiFi takes precedence over Data Roaming because it is faster.
Hope this clarifies my first post.