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The standard with the most momentum at this point is UMA. But to be clear, MobileIP and UMA are not competing at this time. This may require a little explanation, so here goes…
The reason for UMA's momentum is that the most difficult part of mobile providers using dual-mode phones in order to take advantage of the unlicensed spectrum is the persistence of the sessions with their mobile clients. In the early days, many service providers thought that endorsing wireless VoIP network technologies would mean losing valuable minutes on their networks. Answer: figure out how to tightly integrate the local area wireless network with the mobile network. Enter UMA.
Being able to give a user extended coverage through unlicensed spectrum without losing the valuable usage time on the cellular network is the ultimate goal. Recent developments have suggested that mobile service providers have finally embraced the unlicensed spectrum technologies and are working to "get in front" of this potential loss to their valuable minute usage (good news for us because we saw a similar phenomenon happen when cable started threatening voice service in the US and the RBOCs quickly countered with aggressive DSL rollouts). That being said, UMA is a proposed set of protocols designed to standardize the way a GSM and GPRS service gets handed over to an unlicensed spectrum network – like 802.11 – and the service providers can keep track of who's using what. Bottom line, with UMA the unlicensed spectrum becomes a way to provide coverage in areas that are not normally covered by the cellular network.
Mobile IP, on the other hand, is designed to allow an IP device to roam across IP networks and still maintain their home identity. This allows a mobile IP device, PDA, SIP-phone, laptop, etc., to maintain its IP services without re-logging in or re-initiating higher level applications all the time (or at least it's the intention).
Now, recognize the goal of the mobile carrier (don't lose cellular minutes to Wi-Fi networks) and recognize that mobile carriers are not IP-based networks, yet. Which means when you move from the indoor, Wi-Fi (or Bluetooth or whatever unlicensed network) to the outdoor network, you are effectively moving from a VoIP infrastructure to a non-VoIP infrastructure.
Therefore, if you are creating a dual-mode cellular/Wi-Fi handset, the need for supporting Mobile IP isn't there, but the need to support a protocol for switching networks (i.e. from an IP network to a non-IP network) and maintaining the GSM or GPRS services is. You want to look to UMA.
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