No, it's possible, but not easy as with a GSM phone. As you say, you don't really unlock it as much as it must be registered with the carrier. Sprint has been known to accept outside ESN's into their database, though you'll have to walk into a store and talk to someone. I think someone noted that Verizon has done this as well in the past, but it's not common practice. There are even ways to spoof a new ESN if the coded one is blacklisted. It's worth noting, though, that some services might not work quite right if you take one phone to another, but usually just the carrier sponsored apps/services that might be pre-loaded.
And where did you get that Sprint and Verizon are not compatible? They actually operate on the same bands. How do you think Sprint phones roam on Verizon's network?