How close can light fixtures be to fire sprinkler heads?
I was able to track down the relevant code document for residential sprinker installations - it's 2010 NFPA 13D, section 8.2.5.2, and it's actually viewable online for free if you register your email address.  There is a different code for commercial buildings.  For this issue at least, the residential requirements are much more strict.

The residential code says pendant sprinklers must be at least 3 ft from obstructions such as ceiling fans and lights (as measured from their centers), or you can use this table which relates A) the distance from the sprinkler to the near edge of the obstruction and B) height of the sprinkler's deflector above the bottom of the obstruction.

ceiling----------------------------------------------
              |                  *               *
  sprinkler  -+-      ---        *  obstruction  *
                       B         *               *
                      ---        *****************

              |&lt;----    A   ----&gt;|


---------------    --------------
Distance A (ft)    Maximum B (in)
---------------    --------------
up to 1.5          0
1.5 - 3            1
3 - 4              3
4 - 4.5            5
4.5 - 6            7