Looks like the SMS wont be affected if you turn off just data and use the WIFI service for internet etc.
Rim documents this.
Turn off data service or set data service options when roaming.
Depending on your wireless service plan, you might be able to turn off data service (email, PIN and MMS messages, and browser service) on
your BlackBerry® device so that only phone and SMS text messaging are available. You can also set your device to turn off data services when
roaming. For more information, contact your wireless service provider.
1. On the Home screen or in a folder, click the Options icon.
2. Click Mobile Network.
3. Perform one of the following actions:
xxx8226; To turn off data service, change the Data Services field to Off.
xxx8226; To turn off data service when roaming, change the Data Services field to Off When Roaming.
xxx8226; To receive a prompt when roaming that allows you to turn on or turn off data service, change the While Roaming field to Prompt.
4. Press the Menu key.
5. Click Save.
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