Daniel Lehenbauer has posted an update for his 3D tools for WPF on GotDotNet. This version is compatible with WinFX Beta 2 and Windows Vista Beta 2 and adds a community-owned implementation of "ScreenSpaceLines3D".
ScreenSpaceLines3D are 3D line primitives for lines with a constant thickness when projected on your 2D screen. WPF does not contain such a built-in type of line at this time. Lines that do not get "thinner" when viewed from a 3D perspective camera have their use for applications like 3D wireframe views. Daniels update also includes a 3D wireframe viewer.