here's the press release:

London, UK — 3 September 2008— Sony Ericsson today published its Software Developer Kit (SDK) for Xperia™ X1 and invited developers and content creators to capitalize on the opportunities offered by developing mobile content for the phone. Whether it is music, photography, email or instant messaging, gaming, social networking or video applications, developers can now create a host of unique multi-media entertainment and content for the Xperia™ X1 for consumers to enjoy.

<snip> Read more here: http://www.sonyericsson.com/cws/corporate/press/pressreleases/pressreleasedetails/key.PressResource.SonyEricsson_Xperia_X1_panel-20080903

Watch the video here:

 

SDK runs only on Visual Studio 2005 with the Windows Mobile 6 Professional SDK installed.

i tried installing it on Visual Studio 2008 but it failed, with some tweaking (changing the target install path) you could get it to install but the solution fails to deploy to the emulator. I'm on email with the Sony Ericsson dev team to solve this issue. stay tuned.