Sometimes the revolving door spins quickly at the top. AllThingsD reported today that HP executive Jon Rubinstein is leaving the company for places unknown. Rubinstein is perhaps best known for his days at Apple, when he spearheaded the rapid growth of the iPod music player -- the piece of Apple hardware widely remembered as the "gateway drug" that has since attracted millions of customers to the Mac, iPhone, and iPad. After Apple, Rubinstein worked at venture firm Elevation Partners, then moved
