VZW hasn't sold locked phones in over a year. I can't speak to the X specifically, but the new Droids are virtually identical and my Maxx is getting T-Mobile 1900 MHz 4G right now. I'm not sure if or how well it'll work with some carriers (US Cellular and Sprint, for example) based on their unique spectrum use, but if you're going to AT&T or T-Mobile it should work fine. Do some more research and ask whichever carrier you're switching to; T-Mobile helped me with setting up their APN manually, as my Maxx didn't detect the settings automatically.
Nobody seems to have told Verizon's tech support that the new phones are wide open, so they'll still tell you that any 4G phone they sell is internationally unlocked but "incompatible" with US GSM carriers. That's what they did with phones like the Droid RAZR, RAZR HD, and Galaxy S3: in the firmware which manages APNs, any network with a US country ID would be rejected as "unknown," so you'd typically get voice and SMS but no data.