Microsoft spokesperson Aaron Greenberg has said there are no plans to release the 250GB Xbox 360 outside of limited edition hardware bundles.
A Final Fantasy XIII package was announced just last week. Previously, the 250GB unit has been available as part of Modern Warfare 2 and Forza Motorsport 3 bundles, but never as a standalone product. Greenberg told Joystiq that limiting the release of the console is "part of what makes it special".
"We're always evaluating, but for now we feel like the 120 is serving us very well as the standard offering," he said. "And then having the 250 as a limited edition is a good balance."
So there you go.