I just used my iPhone 4S in Tokyo and Kyoto. (June 2013) It works perfectly on their CDMA system there, even though I am an AT&T (GSM) subscriber in the US.
It seems that, unlike the earlier iPhone models, the 4S is both CDMA and GSM compatible. Be careful though, unless you buy a Data Plan from Verizon before you leave the US, the use of their 3G internet access will be really expensive, if you enable "Data Roaming" while traveling there. Also check to see if you can purchase an international calling plan that reduces the per-minute rate while traveling in Japan, before you depart. Do the same for text messaging.