It's tricky. 2008 may have an improved interface for doing this, but I'm not aware of it. Traditionally, judicious use of the SOLVIEW and SOLDRAW commands can produce the desired result - but it can be extremely fiddly.
I can't remember off the top of my head whether or not you can assign dashed linetypes to 3d objects. If you can, you might find it easier just to plot a paperspace view to pdf twice - once with HIDEPLOT set to ON, using a continuous linetype, and once with with HIDEPLOT set to OFF, using a dashed linetype. Once plotted, it would be fairly easy to combine the two views in your favorite graphics package. That's actually how I do it - but I don't use dashed lines. I just make the 'hidden' lines fainter in photoshop by adjusting the transparency of that (photoshop) layer.