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 in relevant places to call to call out considerations for developing for 256-MB devices. If you want to dig into the docs, a good place to start is the overview topic, Developing for 256-MB Devices. If you are developing with the XNA Framework, you can check out a new topic on reducing memory use in XNA games.
Making sure your applications run well on 256-MB devices will provide you with the greatest market reach for your apps, so check out the tools and docs today!