This should be some kind of bug, because Adobe Reader X in MacOs X and Windows
produced the same wrong clipped letters as in the png image that I posted.
I have been reported that Illlustrator Cs5, on Mac and PC produced the same
wrong results.
Illustrator Cs4, Foxit Reader, Nitro PDF, Preview (for MacOs X) and probably many
others programs do not add this extra clipping path to the letters...

Alejandro