In terms of the shortage - part of the reasons are baby boomers getting older, American health getting worse (obesity, diabetes raging rampant).  More woman are entering the medical field, and with that they will probably take some time off for children.  
I also think a factor is individuals' perceptions of what a doctor is.  If you look on TV, people entering med school in the last 5 years and for years to come saw ER, Chicago Hope, and now Grey's Anatomy and Scrubs.  All portray medicine as being hands on, life or death, gory.  That's not primary care.  People 10-15 years ago had Dr. Quinn, primary care physician who lived in the 1970s near an Indian plantation.  To us (or at least me) that seems old school.  I want x-rays, ekgs, gamma knifes.  You're more likely to find the first two at your dentists office than a PCP office.  
People may disagree with tv, but forensic tech jobs have only grown in popularity after shows like CSI came out.  
Lastly - to be a physician on must have a medical license which requires 1 year of residency.  The ACGME has stated it has no desire to increase the number of residency programs for several reasons, but one is to keep the field of medicine an elite profession.  Elite - people want it - meaning demand is more than supply.  I heard there was a study where the others claimed that the US could not copy Massachusetts health care plan because there wouldn't be enough physicians.  Even if people started going to PC, they would still need more physicians as a whole.  
As long as the demand for physicians goes up (baby boomers, diabetics) and the supply of physicians stays the same (# of licensed physicians is strongly related to # of residency spots) there will be a shortage.