Posted from the Amazon forums:
http://www.amazon.com/Kindle-Menu-ve...2LAIYMW441C/1/
I tried myself and had the same problem. If you download the following MOBI file on your Kindle 2:
http://www.gutenberg.org/cache/epub/11/pg11.mobi
And hit menu while inside it, the Menu text is all faded. It's also slightly larger in font size; the box is cut-off at the bottom of the display.
Clearly this is a bug of some sort, but what in these mobis is causing it?