Running clang-format should ideally idempotent, i.e. if I run it,
then run it again, the results of the first and second run should
be identical.

This is currently not the case in the following example:

-- 8< --
int test() {
  int foobar = 0; /* comment */
  int foo = 0;    /* comment */
  int bar = 0;    /* this is a multiline comment
                     and the later vars are shorter */
  int qux = 0;    /* comment */
}
-- 8< --

After running clang-format --style=LLVM on this, you get this:
-- 8< --
int test() {
  int foobar = 0; /* comment */
  int foo = 0;    /* comment */
  int bar = 0;    /* this is a multiline comment
                  and the later vars are shorter */
  int qux = 0; /* comment */
}
-- 8< --

Each run, the line before variable qux gets a little bit less indented. After enough, the process finally stabilizes at this version:
-- 8< --
int test() {
  int foobar = 0; /* comment */
  int foo = 0;    /* comment */
  int bar = 0;    /* this is a multiline comment
and the later vars are shorter */
  int qux = 0; /* comment */
}
-- 8< --

I am using clang SVN revision 205521, llvm rev 205525, on Mac OS X 10.8.5.