Consider this test case:

[test.vala]

class TestParser: Object {
    
    static MarkupParser parser = {start, end, text, null, null};

    void start (MarkupParseContext context, string name,
                string[] attr_names, string[] attr_values) throws MarkupError
    { }

    void end (MarkupParseContext context, string name) throws MarkupError
    { }

    void text (MarkupParseContext context,
               string text, size_t text_len) throws MarkupError
    { }
}

[/test.vala]

For the class init, vala is currently generating:

[class_init]
static void test_parser_class_init (TestParserClass * klass) {
  ...
  test_parser_parser = {
    _test_parser_start_gmarkup_parser_start_element_func,
    _test_parser_end_gmarkup_parser_end_element_func,
    _test_parser_text_gmarkup_parser_text_func, NULL, NULL
  };
}
[/class_init]

Which does not compile:
  test.c: In function ‘test_parser_class_init’:
  test.c:104: error: expected expression before ‘{’ token

Instead, vala should generate:

[class_init]
static void test_parser_class_init (TestParserClass * klass) {
  ...
  GMarkupParser tmp0 = {
  _test_parser_start_gmarkup_parser_start_element_func,
  _test_parser_end_gmarkup_parser_end_element_func,
  _test_parser_text_gmarkup_parser_text_func, NULL, NULL
  };
  test_parser_parser = tmp0;
}
[/class_init]

Which compiles fine.

Actually, vala does exactly that to initialize instance variables.
So, the same logic should apply to static variables too.

And that explains why:
  http://live.gnome.org/Vala/MarkupSample
Compiles with:
  const MarkupParser parser
But not with:
  static MarkupParser parser