Brian Klugg at Anand said the iPhone 5 isn't currently capable of VoLTE and Apple specifically filed the iPhone 5 as *NOT* VoLTE capable in their FCC filing. See the FCC Test article image and explanation here: »www.anandtech.com/show/6295/why- ··· support-
I believe VoLTE is simply a baseband software upgrade however, but whether Apple decides to enable that when the carriers support it is up to them. If Verizon and AT&T support VoLTE in 2013 as claimed, I believe that Apple will update the iPhone 5 for VoLTE as long as it is not a hardware limitation. Anand explained that when you buy an LTE SoC from Qualcomm, you pay for features, even if they go unused. So it's completely possible Apple didn't buy the VoLTE "feature" when they negotiated the license for the baseband chips. He stated the baseband was all the same and the features were enabled at the software level however (in the Qualcomm provided SDK I believe) so I don't see why they couldn't negotiate to have it enabled at a later date.
As for why most Android phones can do simultaneous CDMA and LTE, as others have stated they either have two radios (and subsequently suck battery life) or in the case of newer CDMA/LTE SoC that are combined onto a single die (ala Samsung Galaxy S3) they have dedicated CDMA/LTE antennas. Usually one more than the iPhone 5.
So the bottom line is that if simultaneous voice and data is critical for you, get a phone that supports it or don't go with CDMA service.