Well, you are not correct that Verizon's Icon is not compatible with ATT. To best of my knowledge, the Icon is sold unlocked. From Verizon's
specs page:
Network
- LTE Band 13 & 4;
- CDMA 850/1900;
- WCDMA 850/900/ 1900/2100,
- GSM/EDGE 850/900/1800/1900
On WCDMA ATT uses band 2 (1900) and band 5 (850), so this means it will get 3G (or fake US 4G)
On LTE side ATT uses band 2, 4 and 13 which would provide decent LTE coverage, with exception of areas that use band 2
On T-Mobile the Icon would get LTE or 2G (EDGE) speeds.
Having said all of that, personally i would never buy Verizon phone, even if they sell it carrier unlocked with huge discount. They appear to be on completely separate update cycle, so security updates would be rare. People have reported that Verizon have stripped the functionality of WP8 to modify/add apn's. On ATT or T-Mobile MMS would never work, because the MMS apn cannot be added.