During their tear-down of the Verizon iPhone 4, iFixit discovered that the vibrating motor has been moved and redesigned.  Of course, their Tweet on the subject might be misconstrued if taken out of context:

Setting that aside, who on earth cares as much about these details as Apple?  They re-issued a 7-month old phone with virtually the same internals as the original, but were bothered enough, apparently, by how loud, harsh, and "dissonant on a table" the original vibrator was.  As Pixar founder Ed Catmull is fond of saying:  Success hides Problems.  We should all be glad that neither Apple nor Pixar is willing to rest on their past success.

Of course, some would argue that Pixar has had all the success they need for awhile.