Originally Posted by
pinniped
Reality: getting IDB'ed is very, very rare.
According to
http://airconsumer.dot.gov/reports/2...200907ATCR.PDF (page 30), the IDB rate for January to March 2009 was 1.31 per 10,000 passengers.
Since the OP mentioned Newark, the rates for Continental and Express Jet were 1.42 and 2.39 respectively per 10,000 passengers.
However, your risk of IDB is likely higher if you (a) do not have a seat assignment (on airlines with pre-assigned seats), (b) do not have elite status, and (c) check in later rather than earlier.