I know BPDs are famous for idealizing and obsessing over people they actually know, but for me, it's with celebrities.  I'll develop an intense crush on a celebrity, watch all their movies/listen to all their music, learn everything I can about them, and generally surf the Net obsessively researching them.  The obsession can last anywhere from a year to several years.  In the past, it's been Brad Pitt, George Michael ... and now it's swimmer Michael Phelps.  

It's like I'm a schoolgirl all over again with boy band posters on the wall or something.  I realize how silly it is and don't talk about it much with others, although my closest friends have all learned to recognize this idiosyncracy of mine by now.