I was just wondering if I bought either an iPhone 4 or iPod Touch and then later next year the iPhone came to Verizon if the apps would be transferable to Verizon's iPhone.
Probably the software is compatible, but different carriers lock down certain features. For example, skype will probably work only on 3G for verizon and use your minutes.
Theoretically (since the iPhone coming to Verizon is still entirely theoretical), apps should work across both platrforms since iOS4 would still be the OS regardless of CDMA/GSM carrier....but I guess we won't know until if/when the iPhone ever comes to Verizon (which may or may not ever happen).