US Cellular will not activate any device that didn't originally come from them. What do you mean "flash it"? In order for this to work on CDMA, the ESN would have to be added to the carrier's system and then the SID and MTN would have to be changed on the device.
Refusing to activate better devices that they don't sell seems like shooting themselves in the foot these days.
Either way, every person I know who has done business with USC does not like them. Usually it has to do with getting billed for things they didn't do and the carrier refusing to remove the charges. I know one person who cancelled service and they kept sending him bills for 6 months.
This isn't entirely true. But you have to know what you're asking for.
US Cellular uses CDMA, therefor they can activate any CDMA phone on their network provided the ESN is in their system. Verizon and Sprint ESN's are not in their system. You can get them to add it, but you're going to go through a whole lot of hell to succeed. The reason is that wireless carriers generally don't add foreign ESN's except in very limited cases (Car phones mainly) where the device simply isn't replaceable with one of their own.
Where I worked (not US Cellular,) occasionally someone with an old car phone or suitcase-sized phone would call up to try and activate them. Usually we'd just play dumb, try and activate it and when it fails tell them it's not supported. However there is usually an escalation procedure where a technician can actually add the ESN to the HLR manually, thus bypassing the billing system's ESN validation. Note I'm referring to a 2G TDMA system not a 2.5G CDMA2000 or GSM system. In the older CDMA and TDMA systems, the analog mode ESN could be activated on either. When the Analog system was shut down, obviously this could no longer be done (TDMA on CDMA or vice versa.) However the ability to enable an alien ESN still is doable, you just have to find someone who is aware of it and is willing to sacrifice their performance metrics to do it for you.
That said, why would you buy a Verizon or Sprint iPhone and then use it on another carrier? It's not like the GSM devices where the actual device is irrelevant, the network only cares about the IMEI on the sim card. The device may be sim-locked, but you just need to acquire a manufacturer unlocked phone from another country (eg Canada.)
http://store.apple.com/ca/browse/ho...iphone/iphone4s?afid=p219|GOCA&cid=AOS-CA-KWG
"About the Unlocked iPhone
The unlocked iPhone includes all the features of iPhone but without a contract commitment. You can activate and use it on the supported GSM wireless network of your choice.* The unlocked iPhone 4 or iPhone 4S will not work with CDMA-based carriers."