I haven't verified this yet, but here's more investigation from our star CAD tech:
"If you use AutoCAD with wipeouts or textmask and plot to PDF’s you may have seen big black boxes or lines where the textmask and wipeouts are, on you plotted your pdf’s. Here is the solution with AutoCAD 2008:
Toolbar: Draw
Menu: Draw > Text > Multiline Text
Command prompt: Mtext
In the text entry or text editor dialogue screen right click for the task related pop-up and select “Background Mask.” Check the box for “use background mask” and check the box for “use background drawing color.”
Note: when looking at the model from a white sheet’s viewport, the background mask’s color stays the same as the model (black usually). Fix: if it bugs you, change your sheet background color to be the same as your model background color.
More information:
Use Background Mask: Puts an opaque background behind the text. Note that when you apply a background mask to Mtext multiple columns only the column areas will be masked. The space between the columns commonly referred to as gutters will be unmasked.
Border Offset Factor: Specifies the margin around the text for the opaque background. The value is based on the text height. A factor of 1.0 exactly fits the multiline text object. A factor of 1.5 extends the background by 0.5 times the text height.
Fill Color: Specifies the color for the background. Use Background Drawing Color. Provides a background that is the same color as the background of the drawing.
Color: Specifies a color for the opaque background. You can select one of the colors in the list or click "Select Color" to open the Select Color dialog box."
As I said, I havent't vverified any of this, but it will hopefully lead to your own solutions.