In my evolution course, I note that "Darwin spent 20 years working out his ideas and gathering evidence" before releasing On the Origin of Species in 1859. I don't say he "delayed" publication purposely, though in many cases this long period from idea to outcome has been attributed to fear of the reaction from the clergy, colleagues, society at large, his wife, etc. On this issue, a few bloggers have pointed to a recent essay by John van Wyhe (2007), in which it is argued that there was no delay based on fear, only a protracted writing period. Other historians do not necessarily agree, though the blogs I saw did not mention this. As Odling-Smee (2007) says,
…several Darwin scholars are not convinced. Kohn and others agree that the way in which cultural and social pressures influenced Darwin's decisions may have been overplayed, particularly in the public arena, with less attention being paid to the involved process of scientific discovery. But the consensus in the field is likely to remain that a multitude of factors underpinned Darwin's delay.
Kohn points out that searching for explicit references to a "delay" is a simplistic approach to the problem, and that other factors should be considered. For example, Darwin often criticized religion in his notebooks, which suggests that he would have been aware of the probable implications of his theory for religion. It is hard to see how the absence of specific references to a delay rules out any influence of cultural and societal factors on Darwin's decisions, agrees David Quammen, author of The Reluctant Mr Darwin.
Kohn also points out that in Darwin's later publication The Descent of Man, which applied the theory of evolution to humans, Darwin specifically states in the opening lines that he delayed publishing this tome until he was convinced that the climate was right. It seems likely, therefore, that he would have been aware of the controversy his theories would cause from the outset, and probably avoided discussing humans in Origin of Species for this reason.
I think "yes or no" to the question of whether Darwin delayed publication out of fear is very simplistic. Anyone who has written anything of substance knows that sometimes the effect of fear of reaction is procrastination and/or excessive desire to include every piece of information available. Both can cause writing to take longer than it otherwise would. Was Darwin thorough? Yes. Is that one reason it took so long? Undoubtedly. Was he so thorough because of a fear of reaction? Probably at least in part.
Odling-Smee, L. 2007. Darwin and the 20-year publication gap. Nature 446: 478-479.
Van Wyhe, J. 2007. Mind the gap: did Darwin avoid publishing his theory for many years? Notes and Records of the Royal Society 61: 177-205.