The Salt Lake Tribute recently wrote a story about Mormons leaving Salt Lake City and relocating into the suburbs. It is true that many members of the Church are leaving the city into more affordable housing in places like South Jordan or Lehi. However, Church membership remains strong in Salt Lake. Only two stakes have ever been discontinued in Salt Lake: one in Eagle Gate (2003) and the other at the University of Utah (the latter was dissolved in 2004 and a new stake was created in 2007 to replace it. It was due to a new institute building being built). I do find it likely that some stakes will be combined in the coming years due to the continuation of members leaving the city. The Church has been successful at bringing people into the Church in the area (especially from minority groups) but has not been at as fast a rate as people moving away.
I went back into my data from 2004 when I started to get seriously interested in the growth of the Church and Church demographics and found that the number of wards in Salt Lake City (among stakes that are not Tongan or have "Utah" in the name) have decreased by 10 and branches have increased by nine. This is out of a total of 60 stakes. Many of the new branches created are Spanish speaking. This illustrates that members are not all leaving at once, but is a trend. Also, it shows that members are still moving to Salt Lake as well.
Here is the url for the story