Hi Chad and everyone,
The real question here (I think) is the reliablilty of the Greek manuscripts from which the Bible versions were translated from.
The KJV and the NKJV are translated from the Majority Text also know as Textus receptus, Received Text or Byzantine Text, etc.
The NIV and NASB and some others are translated from newer Greek Editions which actually are derived from more ancient manuscripts than the Majority Text, some dating back to the 2nd century.
We know for absolute certain that the Majority text has been handled a lot by many people, some of whom edited and altered some verses. (This fact is undisputed.) For instance Martin Luther added some words that we know of, and the Dutch theologian, Erasmus, under much political pressure added the section in 1 John 5:8. We know too that Junia's gender was changed in the 13th century by a monk.
Having said that, the words that were added are mostly in line with Christian theology and pose no real theological problem, in fact I can imagine that the new inclusions were done by some well meaning (though misguided) people who wanted to emphasise some wonderful truth. So its not really that the NIV has deleted certian verses (and there are quite a few verses involved in this) it's that people progressively
added them to the Majority Text used in the KJV.
It is also important to note that King James himself had some real personal agendas that he wanted addressed in the new translation that he authorised. (You can easily check these out on a webiste.) The KJV is not the perfect version that most people seem to think it is, the translators were well aware of their shortcomings and it had to be revised many times. Also the KJV was not the first English bible translation. (Some people seem to think that because it was the first it must be the best, but it was neither the first or the best.)
I am not a fan of the NIV because it is a fairly dynamic translation - not particularly literal. If you want a more literal translation of the most reliable Greek manuscripts of the New Testament, I don't think you can go past the NASB. (The New American Standard Bible.) In this Bible any words that needed to be added for clarity are written in italics so you can more easily see what the original authors wrote, and wa.
(The Message is not a Bible translation but someone's very personal interpretation.)

evangeline