The IP eXchange (IPX) is used for data when the users roam between different networks. GPRS Roaming eXchange (GRX) is a service within IPX. One of the main areas of discussion within the LTE World Summit 2012 in the Signalling Focus day was roaming on LTE. Different vendors have different proposals and solutions; couple of them are as follows:
Interesting to see that iBasis has proposed LTE Signalling eXchange (LSX) as a way forward.
A presentation from Acme Packet (for an earlier conference) has interesting VoLTE roaming options proposal.
Finally, while everyone was focussing on LTE-LTE roaming, only Diametriq was looking at LTE-LTE/3G/2G Roaming. The relevant part of their presentation is embedded below.