They already do, the Blackberry Tour supports both CDMA and GSM (via SIM slot) the SIM isn't required for pure CDMA operation. It was used to allow CDMA devices to roam in Europe where there was no CDMA.
As far as frequencies go, 850 is standard for North America, and infact is the same frequency Verizon is using for CDMA, they are both the old A band (ATT) and B band (Verizon) that was used in analog in North America for years, as far as the antenna goes, both support 850 and 1.9 which are the same frequencies used in most carriers in North America today. AWS is the new one that isn't supported yet, actually when I was at CTIA last year and asked Qualcomm, they expect AWS support sometime this year (2011).