From what I understand, the iPhones sold for GSM networks (e.g. AT&T and T-Mobile) use an Intel chipset for the modem that does not include CDMA capabilities. Those sold for Verizon and Sprint contain a Qualcom modem that has both GSM and CDMA capabilities. Press reports indicate the Intel modems may support new technologies expected to be rolled out by AT&T and other carriers in the foreseeable future that phones with the Qualcom chips will not be able to take advantage of.
This may come into play when considering which way to go. I can't see it becoming a real issue for at least a year if not two give the speed at which AT&T and other carriers manage to implement changes, but who knows...
You're absolutely correct in that purchasing a Verizon phone will provide maximum flexibility right now. Who's to say what the future holds...