I have been using Cisco WIP310 for a while now (in a small scale home environment)
This phone connects to your network over WIFI connection (802.11 b / g) and supports SIP protocol.
I use this phone in an area with only one WIFI access point, not sure if it would work ok over longer distances where for example you would like the handset to connect to an access point nearby. This would be ideal if you are moving around in a large area.
Check out http://www.cisco.com/en/US/prod/collate ... 02697.html
One big disadvantage is that Cisco no longer sells or supports these phones, so for a production environment this may be a bad choice. You can still buy them on Ebay for example. I have been looking for wireless phones that connect over WIFI and support SIP for a long time with very little succes. If you happen to find any good WIFI/SIP phones let me know!