Ok I just found the solution. The problem occurs when you are using True Type fonts in Vista.
TO fix this on your icon for starting ACAD right click and go to properties. Choose Compatibility tab. Uncheck all except the Disable Desktop Composition...
I have this exact same problem except I am using AutoCAD 2009.
The font disapears in the viewports when I change it from Arial to Times New Roman. When I zoom out like you said it becomes visible. See if the font is visible for you when you change it to Arial. This is very annoying.
