This isn't that surprising. When SE released the Windows Mobile-based Xperia X1, it did so in two versions: The X1 (outside the U.S.) and X1a (U.S.-only). And what do the X1 and X1a still have in common? Neither was picked up by a U.S. carrier, leaving the hard-core (and deep-pocketed) shelling out $800 for a nice piece of hardware with a so-so UI.
Will that happen with the X10? Or will it wind up on, say, T-Mobile? Stay tuned, folks.