WordArt is not allowed in Shared Borders because it places code in the
<head> section of the page. In a shared border, any code in the
<head> section will not be copied to the pages using the shared
borders - hence the wordart will be invisible, or worse corrupted.
The best way to use WordArt anywhere on the page is to paste the
wordart into a graphics editor, crop and optimise the image and save
as a .gif or .jpg. Import the image into your web, and use that
image.
WordArt, TextBoxes, ShapeArt and other objects from the drawing
toolbar use VML graphics. These are absolutely positioned (which will
cause severe headaches in left, right, and bottom borders), and the
images can only be seen in IE, other browsers will show nothing or
very poor quality .gifs.