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There are already a number of endpoint devices that are capable of supporting WiFi and GSM.
http://www.motorola.com/wlan/docs_pdf/cn62mod.pdfhttp://www.gizmodo.com/gadgets/cellphones/nec/docomo-nec-900il-025631.php
These phones support SIP for VoIP when in range of a WiFi network and also support GSM.
This was first published in July 2005
