WhatsApp — the incredibly popular multi-platform smartphone messaging app — has stepped up its strategy to tempt mobile subscribers away from SMS, signing its first carrer roaming deal with Hong Kong operator Three to offer unlimited use of its app both domestically and in 78 other countries for little cost.

As part of the deal, Three’s Hong Kong customers will be able to gain unlimited access to WhatsApp for HK$8 (US$1) a month, ensuring that any data incurred using the app is not deducted from their existing plan.

However, it’s the global ‘WhatsApp Roaming Pass’ that looks set to upset the likes of RIM, with the offer of unrestricted access to the messaging service in 78 destinations around the world for a HK$48 (US$6) daily rate. It’s WhatsApp’s first roaming deal, and from

Paying $6 a day, users can effectively drop any of the additional data packs that carriers offer, relying on WhatsApp to do the heavy lifting when it comes to messaging and photo uploads. Users will be able to request that their friends and family download the app (if they haven’t already) to keep in contact, instead of using traditional (and costly) SMS methods.

The service emulates RIM’s BlackBerry Messenger service, which allows BlackBerry owners to send and receive an unlimited number of messages worldwide for free with no charge. The difference here is that Three customers will be able to interact with iPhone, Android, Windows Phone, BlackBerry and even Nokia handset owners.

With Vodafone and Hutchinson powering Three’s Hong Kong service, it’s undoubtedly leaning on the two companies’ existing infrastructure and subsidiaries worldwide to provide connectivity.

It’s a significant deal for WhatsApp, but the company is already pioneering the messaging space in its own right. The company already has over ten carrier deals in place worldwide and now facilitates over 10 billion messages a day, connecting users regardless of their handset.