** UPDATE ** A December 2007 CTP is available, more info here.

Looks like we will have to wait a couple of weeks for WDP support in Visual Studio 2008 as noted by Scott Guthrie:

"Two popular add-ins to Visual Studio are not yet available to download for the final VS 2008 release.  These are the Silverlight 1.1 Tools Alpha for Visual Studio and the Web Deployment Project add-in for Visual Studio. Our hope is to post updates to both of them to work with the final VS 2008 release in the next two weeks."