Before I get to the statements on this issue that have been stated by 
more "official" spokesmen than myself, The first thought that I had when I had 
read this question, was that people always have their free agency. 
 
The first example I had thought of was Jonah and the city of Ninevah. (Jonah Chapter 3) 
He had prophesied that the city would be destroyed (vs. 4), but later was angry 
because the LORD didn't destroy it as he prophesied, because they had repented. 
 
The Lord will never take away our choice to choose, whether that choice be good or bad. 
 
Now here are some more substantial answers than mine above. 
 
President Joseph Fielding Smith wrote: “There have 
been various interpretations of the meaning of 
a generation. It is held by some that a generation is 
one hundred years; by others that it is one hundred 
and twenty years; by others that a generation as 
expressed in this and other scriptures has reference 
to a period of time which is indefinite. The Savior 
said: ‘An evil and adulterous generation seeketh after 
a sign.’ This did not have reference to a period of 
years, but to a period of wickedness. A generation 
may mean the time of this present dispensation.” 
(Church History and Modern Revelation, 1:337.) 
 
The Lord was speaking of the temple in Zion, that is, 
Jackson County, Missouri. The Lord later excused the 
Saints from building that temple because the mobs 
prevented it (see D&C 124:49–51) and because the 
Saints at that time had not kept the commandments 
as they should (see D&C 105:1–9). 
The day will come, however, when the holy city of 
God will be established in Jackson County, Missouri, 
and the temple will be filled with the glory of God as 
foreseen by the prophets. 
 
P.S. Look up those Scripture references on lds.org, for the D&C ones. The answer there comes straight from the LORD.

Here is a place that has a very extensive answer to the broader question of Joseph Smith and prophecies: http://en.fairmormon.org/Joseph_Smith/Prophecies 
Fairmormon.org

Prophets are not infallible. Some thoughts: 
 
- A prophet himself is not infallible. He is human. He has opinions and thoughts that are not always right. But when he speaks as a prophet - then we can know that what he says is true. The important distinction is to know when he is speaking as a prophet and when speaking as himself. 
- Many of the comments that some people say are reversals are really the result of mis-quotes, things taken out of context, etc. 
- Here is a great article talking about what is doctrine:  
Mormon Doctrine 
 
So the question is when is the Prophet Speaking as the Lord? Here is a quote from one of our scriptures. It is in Doctrine and Covenants Section 68:  
3 And this is the ensample unto them, that they shall speak as they are moved upon by the Holy Ghost.  
4 And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the bpower of God unto salvation.  
 
God definitely is infallible. We His children - even including His prophets - are not infallible. That is one of the reasons we are here on earth is to learn and experience life and try to become more like God and to slowly and steadily become better and better. 
 
Lastly let me also say that the Prophets and Apostles (esp.those in our modern day) are some of the best and most righteous men who have ever lived on the earth.

You are right Joesph Smith did say that a temple would be built in Zion in his generation (D&C 84:8).  
 
Let's look at a prophesy of Christ in Luke 21:31-32 "So likewise ye, when ye see these things come to pass, know ye that the kingdom of God is nigh at hand. Verily I say unto you, This generation shall not pass away, till all be fulfilled." 
 
Here Christ says that all things pertaining to the return of Christ shall be fulfilled in the Apostles generation.  
 
So is Christ a false prophet too? 
 
Obviously not! The only logical solution is that Christ does not mean a generation in the terms of man but is referring to the entirety of mankind as a generation. Which makes sense from a celestial position since Christ is eternal and all the generations of man are insignificant in the scope of eternity.  
 
Thank you for your question I hope that I was able to answer it well!