When using Ubuntu font many users see rectangles in different places, that don’t belong there. They come from the .notdef character which in that very font has a rectangle shaped outline. This character appears when the font doesn’t contain a character for the codepoint requested. It seems like some apps use codepoints different from usual spacing characters to show empty room: e.g. with launcher pro in the app drawer long names are shortened with an ellipsis after which those squares appear. The reason why using other fonts doesn’t show these squares is, that some fonts only have an empty space at .notdef, so no outline appears. One can verify this by using the DroidSansNotdef.ttf which I attach. There I’ve only added an outline to the .notdef character and you’ll get the same behaviour as with the Ubuntu font.
UbuntuAndroidfix.ttf has an empty space in .notdef so it isn’t visible. Nevertheless, this is only a workaround for a bug that’s somewhere else.
Edit: As shown below, the real reason is missing U-FEFF in the font, so it is a font bug. Attached you find all four styles (Ubuntu regular, italic, bold and bold italic) with the added character.
20.4.12 — New version: added spacing characters U-2000—U-200F to fix some more box-char appearances.