We visited Buthan from the 1st to the 17th october 2010. We could use the Blackberry service (8900, Swisscom) in this places:
Network: TASHICELL – EDGE or GPRS
Places with Blackberry: Paro, Haa-Valley, Tigernest, Thimphu, Punakha, Wangdue, Phuntsholing
Druk-Trek from Paro to Thimphu: sometimes GPRS
Places without Blackberry: area of Bumthang (Jakar)
The network B-MOBILE does not support the Blackberry service. GSM and SMS is ok.
You have to check, if your mobile company has roaming contracts with TASHICELL and B-MOBILE.
Conclusion: Your provider needs a roaming contract with TASHICELL. Blackberry service is only available in the TASHICELL network in the western part of Buthan.