This topic pops up every once in awhile.  It is good for believers to be
students of the Word and to defend it's purity and authenticity.  So last
night I took the time to read the entire preface to my NIV Bible.  Not going
any deeper than what I read on those pages it would appear that there is no
corruption intended (at least not in a malicious way as to pervert the True
Inspired Word of God).

The NIV uses the Hebrew version of scripture as often as possible in order to
be as close to the original as possible.  Sometimes the Hebrew language
(especially the written language) is unclear as to the meaning.  In these
cases other texts were consulted in an attempt for better clarity.
 
There are explanations for certain omissions.  Sometimes the earliest known
manuscripts do not have these particular verses or phrases that have been
omitted from the NIV.  But usually, there is a footnote to explain.

If you are concerned about the NIV's authenticity, I suggest you research the
story of its development.  I started by merely reading the preface in my NIV
Bible.  This may not answer all our sincere concerns, but it helps give us a
better picture.