If you can, get a Nokia E72 unlocked. If you can't get the E72, get any E series Nokia phone (I have E71).
Reason for recommendation:
* WiFi roaming is painless
* 1500mAh battery: WiFi *drains* battery. You absolutely need the phone with largest battery pack or you're looking at charging it twice a day. A large screen android/iPhone is fun for a week till you get tired of looking at battery bars. Nokia will last whole 3 days on GSM and will get you through the day on WiFi.
* Integrated SIP with same dialing/receiving experience as a GSM call
* VoIP apps: Pretty much every VoIP app is available including Fring, Talkonaut and Skype apart from integrated SIP
* Excellent sound quality
* Small screen by today's standards (you get battery life in return)
* Abysmal inbuilt browser (you can have Opera Mobile and Opera Mini instead)
* It's not hip in US (however, if you want nerd points it'll score many - run wordpress on your phone with downloadable port of Apache2, MySQL4 and PHP5 - no kidding)
* Custom development is painful, but you get everything and the kitchen sink to write apps for the device (Python, Java, C++,
* No touchscreen