I have previously blogged about using the iPhone 4 in Bhutan. To repeat the bad news, in a nutshell: Both TashiCell and B-Mobile do not offer the new micro-SIM cards required by this new phone.
But now there are also good news: If you're in Thimphu, it is now easy to convert ordinary mini-SIMs to micro-SIMs. How? I happen to have a micro-SIM puncher that will easily do the work. So if you put your hands on an iPhone 4 (or an iPad 3G, for that matter) and need to plug in a local SIM card, just let me know (email@example.com).