See MPEP Section 1503, and Section 1504 relating to a requirement for ornamentality.
"Entirely" in scare quotes on account of some ornamental features require text for description, usually surface treatment or texture. The design patents that I have prosecuted sometime contain elements that are not part of the claimed design, for context. This practice is usually accompanied by some text disclaiming the contextualizing material as not part of the claimed ornamental appearance.
The Apple design patents that I looked at just now (D618,677 and D618,678) didn't contain a logo or any text (text being a necessary prerequisite to font), and illustrated the look of an iPhone. It did include width of bezels on the display face, slots, earphone port, and buttons, but didn't include any display objects.
The design patent that really got my goat must have been a different one.
If I recall correctly, the design patent of the iPhone and the iPad were based on being a slab with a singular lack of font, logo, or ornamentation. . . and the black, curves, etc. were part of the design esthetic. . . and it was ruled the description applied. There were font, logo and placement of same on the obverse IIRC, But I think the back layout has a separate design patent.
Thanks for the explicit detail on the Design Patents. I had read that merely adding a logo, or changing a font or manufacturer's name was not sufficient in case law to differentiate a non-infringing design.
Apple insists that no carrier may place any marking on the face of the iPhone or iPad. . . no logos at all.