Wow, quite a comment for a post with multiple technical errors.
"3G works on top of IPv6"
WCDMA networks (such as Cingular's) don't even use IP for data routing within the network.
CMDA network (such as Verizon & Sprint) use Mobile IP for packet data routing but not for voice. It is not IPv6.
"it provides the same service, uses the same technology"
The radio access technologies are completely different.
3G networks have defined core network architectures, WiFi does not even specify this.
"3G works in the 5GHz band"
In US, 3G services operate in 850 and 1900 MHz bands.
In Europe, 2100 MHz is used.
If you had bothered to read thru the wikipedia article you linked to, you would have seen the above frequency bands listed.
WiFi is NOT a threat to cellular carriers as there can be no guarantee of service quality when using the shared ISM band as you cannot control who will also be broadcasting in that spectrum and therefore who will be interfering with you.
Your comment about "I can keep my connection" may work for your Outlook client using session-less https.
However, try setting up a VOIP call (or even an ftp session) and see if this handoff is seamless (it won't be).
The reason is you must completely fall off of the current WiFi access point before trying to find another.
With a cell network, the mobile constantly monitors neighboring cells in the network and move to a better cell when it sees one is available.
This does not happen with current WiFi gear (a,b,g,n).
Are the WiFi standards moving towards solving some of these handover issues? Yes.
However, if and when WiFi Access points support this, they will become more dependant on network configuration and mgmt.
This will add cost and actually make them more like the cellular networks they are trying to replace.
As another poster mentioned, WiFi access points may end up augmenting the current 3G cellular networks (this is actually in the 3gpp Rel6 specs) but will not replace them.
What is a threat (or at least a potential threat) to cellular carriers is traditional circuit-switched services (such as voice) moving to packet based networks.
Currently, cell carriers have no way to recover lost revenue if voice calls move to application level VOIP-type services such as skype.
To combat this, cell carriers are working to introduce the IMS (IP Multimedia Subsystem) which use SIP-based protocols to setup all call services.
This way the carriers can migrate to a packet based network while still being able to charge for individual services.
This is no different from what the traditional fixed-line voice providers are going thru now that cable/DSL providers can offer packet based VOIP services.
However, the above has little to do with what air-interface technology is used (be it WiFi, 3G, WiMAX, or anything else).
Will the switch to SIP-based packet services by the cell carriers benefit the consumer?
Hard to tell.
In the short term, probably not, as it will likely still be cheaper to use services such as skype.
However, as these new services get fully rolled out, competition between cell carriers (and potentially others such as fixed-line or cable carriers) will eventually drive the prices similar to what is offered by skype-like services.
The advantage will be that now a standardized technology (SIP) will be used instead of many proprietary ones (such as skype) which should, according to slashdot philosophy at least, lead to better interoperability and a wider range and lower cost of supported products.