Here is a list of the best wireless mobile devices that you can use today with most SIP services out there:
0. Nokia N80 Internet Edition (just make sure you get the Internet Edition)
1. Nokia E61 after upgraded to its v3.03.x firmware
2. Nokia N770 or N800 with the Gizmo client
3. Any Win2003+ Wi-Fi PocketPC with 400 Mhz CPU with SJPhone client
4. Palm Lifedrive with Articulation client
5. Nokia E60 and E70 *if* Nokia updates their firmware to the v3.03.x version any time soon