Evolution of long necks in giraffes
There seems to consensus that it is not competition for tall food.  Giraffes actually often feed on resources that are lower than their maximum possible height.    See: 

Simmons, R. E. &amp; Scheepers, L. 1996. Winning by a Neck: Sexual Selection in the Evolution of Giraffe. The American Naturalist 148: 771–786

This paper put forth the idea that sexual selection is the reason behind long necks.  The idea is that longer necked males are dominant.  But this  theory has also been questioned.  See:

G. Mitchell, S. J. Van Sittert, J. D. Skinner. 2009.Sexual selection is not the origin of long necks in giraffes. Journal of Zoology 278, Issue 4: 281–286

So in the end there's no clear consensus.  Some papers have returned to the theory of competition for research in the past few years.To put it simply, no there is no consensus.