In AutoCAD, you can assign a non-continuous linetype to a 3D polyline, but it will not display or plot.
If you need 3D polylines, and you need a non-continuous linetype, for example to represent the flowline of a ditch - there are a couple of workarounds. Both assume that you have assigned the correct linetype to your 3D polylines.
This is one of the new commands in AutoCAD 2008 that is sort of overlooked, but can save you a lot of time. If you use xrefs, then how many times have you had a problem with the parent drawing crashing? You might try running AUDIT or RECOVER on the parent drawing and find there are no errors, but you continue to have problems with this drawing. Read more about RecoverAll
Starting in AutoCAD 2008, you can now control more layer properties on a per viewport basis. You could always freeze layers per viewport, but now you can change color, linetype, lineweight, and plot style. Now the same line can be red with the hidden linetype in one viewport and green with a continuous linetype in another viewport.
Since the DWF format has been gaining steam over the past couple of years, one of the requests I have seen a lot is for the ability to automatically create a DWF at the same time you save or close a drawing in AutoCAD. Starting with AutoCAD 2008, you can now do this. Read more about AutoCAD 2008 - Automatic DWF Publish
In previous AutoCAD versions, if you wanted to create a table that was linked to a spreadsheet, you had to either use an OLE link or use a 3rd party tool. New to 2008 is a "Data Link" feature. This almost certainly replaces OLE linked objects, and may or may not replace 3rd party tools.