If you are moving around that fast you should soon be thinking of VoIP using WiFi - which will give you one number, voicemail etc. and some really cheap calling prices. Handsets are coming in quite fast now, eg
this from Netgear. Nokia's E-series phones also support WiFi and can be used with VoIP.
I went to the GSM forum and exhibition in Barcelona in Feb, and the mobile operators are sh!tting themselves that they will lose almost all their roaming revenue.