LTE will allow both voice and data simultaneously on Verizon's network. Data flows through LTE while voice goes through CDMA. However, if LTE is not present, the phone backs off totally to CDMA, and yet, this presentation slide obtained by Phandroid states that both voice and data will be supported.
How? It is most likely due to a modification to the CDMA standard, SVDO, which was first announced in 2009 by the CDMA Development group. The press release announcing it then noted it to be a "device enhancement," but no devices supporting SVDO have been released since that time.
This means the SVDO change is probably being added to the HTC Thunderbolt, which will be Verizon's first LTE handset. However, Verizon has asked its sales people to keep the feature quiet.
Another sad tidbit from the leaked image: the HTC Thunderbolt will not support Verizon's mobile hotspot service at launch.