Making good on an announcement last week, Sprint has said that it will begin selling a dual-mode CDMA/WiMax wireless modem on December 21. The modem will be able to access the Clear WiMax network in cities where the network is active, such as Baltimore. Everywhere else, it will use Sprint’s EV-DO 3G network to access the Internet. The U300 USB modem, made by San Diego-based Franklin Wireless, will cost $150 after rebates with new two-year agreement. Data plans will run $80 per month for the dual-mode card. Sprint already sells a WiMax-only card and charges $45 per month for network access.