Microsoft are focussed on getting Longhorn working with DX9 and the effects will be Pixel Shader 2.0 not 3.0. VedBrat confirmed that DX10 is likely to come after Longhorn and not with Longhorn.
Source: http://www.neowin.net/events/?p=68
Consider also that Longhorn is now scheduled for mass availability in the holiday season of 2006.