We recently released an update to the Windows Phone SDK that provides extra tools and features for developing for 256-MB devices. Most importantly, this tools update contains a new 256-MB emulator to test your applications. Keep in mind however, that this release does not include the necessary license to publish your apps to the Marketplace, so you should use it to test your apps for now and plan on publishing them after the final release of this update. You can expect the final release to be happening soon.
We've also updated the docs to help you develop for 256-MB devices. You will find a new icon 
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