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Data on a BB is routed through RIMs servers. BB users generally aren't charged data if the track is intra-RIM.

It's also compressed and encrypted.

I've used BBM a lot overseas, but have never been charged an "international data" rate. So I would speculate that somehow it's avoided.

I think unexpected is referring to ridiculous charges just for sending a text message, yes data would still apply.

Not for BBM messages.