When LTE is an overlay to a CDMA/EV-DO network, the current de facto standard for voice delivery is Simultaneous Voice and LTE (SVLTE). In this arrangement, voice service is deployed as a 1x service running in parallel with LTE data services. For this solution to work, the handset needs to have two radios that are on simultaneously. The problem that is obvious is that the power consumption would generally be higher as two radios are on when the voice call is ongoing. The advantage (and I think its a big advantage) is that the data speeds are not affected by ongoing voice call and at the same time the state machine is simple.
For some reason this idea is not very popular for the 2G/3G evolution to LTE as the reliance will be on the CS Fallback. I had discussed this idea in the LTE World Summit and had blogged about it, you can read more details and comments here.
There is also a recent whitepaper from Huawei that covers these issues going towards VoLTE. Its available here.
Edit 06/10/11: Changed the acronym of SVLTE from 'Simultaneous Voice Over LTE' to 'Simultaneous Voice and LTE' as this is correct and referred to elsewhere.