The IETF SIP Working Group has a sub-group working on SIP Location Conveyance. The current draft is
draft-ietf-sip-location-conveyance
This is proposed as a tool to enable SIP endpoints (such as SIP phones or ATAs) or their proxy upstream to convey the calling party's location to the called party. The imagined applications are:
This standard doensn't try to solve the problem of knowing where the user is. But it would allow a service provider who maintains a registered physical address for an endpoint to add that information before sending the call to a PSAP (Public Service Answering Point, i.e., an inbound call center for emergencies). In this sense, the standard would replace the ALI (Automatic Line Identification) database lookups with a service-provider lookup. However, if the SIP endpoint knew where it was -- say, based on GPS -- then the endpoint could convey that directly using this standard.
The argument on the conference call was that, for emergency purposes, the PSAP would generally trust the information provided by the calling endpoint, rather than what the service provider knows as the location. This is based on the assumption that the calling endpoint may be moved without telling the service provider in the middle.
OTOH, pizza delivery would likely trust what the service provider says.
It appears the standard will define a way to convey multiple locations; so, e.g., the service provider indicates one location, and the endpoint another. Or maybe the endpoint provides multiple locations itself. But nobody knows the semantics for that -- i.e., what it means.
This standards also exceeds the signaling capability of the traditional protocols, like ISDN ISUP or SS7. They don't have a way of conveying location, so this data couldn't be passed through an ordinary PSTN gateway. To make use of it, the called parties (like Domino's Pizza, or the local PSAP) would need to have VoIP interconnection with the calling party's service provider.
Because an intermediate SIP proxy or B2BUA can insert new location information, the standard could be deployed without major modifications to most SIP endpoints. Instead, it could be implemented in the service provider's core gear, which might be BroadSoft, MetaSwitch, Sylantro, Asterisk, etc.
The biggest "upset" from this standard could be the displacement of the ALI database. Who knows whether that'll be accepted.