Unlocked phones on Carrier's

[Gubber22]

I read a thread some time ago that Verizion was going to start letting unbranded phones be used on thier network as long as its meets thier criteria. I called Verizon tech line, and was told they was not aware of this.
My question is : What network in the US if any allows you to pick your own phone to put it on thier network?
Their are a lot of other Windows mobile phones out their that are better then then carrier's carry, Ones that have more memory, display ect.
I am looking at the Sony X1 to purchase and thier are some others even better. I am still running the xv6800 which has poor memory. I have went into Verizon several times and looked at the Pro, but while testing it in the store I was not impressed with the Running memory when looking at it in the store. I am sure it does better then the 6800 but not enough for me to change, I have walked out the store twice. The Pro 2 really seems to be able to do what I would want on memory, but it hasn't even came out yet and Verizon I am sure will not put it in their line up till it has been out a while. So what network allows other phones on their network? I see all these unlocked phones you can buy but what good are they in the US?