Dot graphs and diagrams can contain links via the URL tag.
This tag seems to be ignored starting with doxygen 1.6.2,
that is there is no link generated at all.

Example:

1. Create a file test.h with this contents:

          /** @defgroup test Test
           ** @{
          \dot
             digraph example {
                  // nodes
                  A  [URL="\ref foo1"]
                  B  [URL="\ref foo2"]

                  // edges
                  A -> B [URL="\ref bar"]
             }
          \enddot
           **/

           /** This is function foo1 **/
          void foo1();

           /** This is function foo2 **/
          void foo2();

           /** This is function bar **/
          void bar();

          /** @} **/

2. Create an default configuration file using:

      doxygen -g

3.  In Doxfile, set:

     INPUT = test.h

4. Run doxygen:

     doxygen

Compared to the doxygen 1.6.1 output the following lines are missing 
in the generated html file:

<area shape="poly" id="node1" href="group__test.html#ga888d87a9f8a6e3e80d5c45e838cc0416" title="A" alt="" coords="77,29 76,22 70,15 62,10 52,7 41,5 30,7 20,10 12,15 7,22 5,29 7,37 12,43 20,49 30,52 41,53 52,52 62,49 70,43 76,37"/>
<area shape="poly" id="node2" href="group__test.html#ga2d19baf875d1256bf3bfe2b8d02f20ea" title="B" alt="" coords="77,125 76,118 70,111 62,106 52,103 41,101 30,103 20,106 12,111 7,118 5,125 7,133 12,139 20,145 30,148 41,149 52,148 62,145 70,139 76,133"/>
<area shape="poly" id="edge2" href="group__test.html#ga49a4b11e50430aa0a78de989ea99e082" alt="" coords="44,54 44,87 39,87 39,54"/>