This is a new feature that the network operators are getting more interested in. The problem is that the Network side is more prepared than the handset side which still has some way to go.
The possibility of sharing part or all of the network by two or more separated commercial entities was not considered in the initial specification work of 3GPP. However, as e.g. a result of partnerships, the need for two or more operators to share common network infrastructure has become an economically desirable goal. Meanwhile, changes to public network operating licence conditions make such sharing possible from a regulatory point of view.
Some work has already been carried out in this area with the definition of the equivalent PLMN concept and partly with the introduction of Iu-Flex, but there is still the need to consolidate these activities under a coherent work plan.
Network sharing is in a way similar to what is done by MVNO's who use the host network to offer services. In network sharing case the other operator would not generally be a MVNO and probably the equipment would belong to both of the operators along with the cost and revenue earned. This is easily possible if the network is not overloaded (as in case of 3 UK) but if its a long time existing operator (like Vodafone) than they may not have enough spare capacity to allow someone else to share their infrastructure.
3GPP Specs for further reference:
3GPP TS 23.251 - Network Sharing; Architecture and functional description
3GPP TR 22.951 - Service aspects and requirements for network sharing