If SIP is used to do call establishment, most of the challenges will be handled through IP layer. Potentially, the network architecture can get around from using Mobile IP. Yet, I believe with full support of mobile IP in the voice network and in the LTE network will be ideal.
Absolutely, to get unique IP end to end, IPv6 is necessary. Even so, there might be transition from one IP network to anther which will require Mobile IP and, potentially, SIP infrastructure to deal with. For IPv4, IP end-to-end can be achieved through NAT and private IP addresses. My smartphone is currently getting private IPv4 address. NAT will certainly happen somewhere.