I have read about the CON ROM for those on the latest ICS leak, that during installation One chooses cdma/LTE or GSM. If one chooses GSM, then only GSM will work, and work very well. If one chooses LTE, then one can get limited GSM via the hidden menu like on stock GB or ICS.
I have NOT read about successfully being able to switch between LTE and GSM just by switching sim card and settings changes, and have everything work: voice sms, and data.
Yes, it seems the hardware can do it, but the SW needs improving.