Adding an identity field to be the new cluster of a large table - how do I get the "right" order?
I just performed the following experiment on a SQL Server 2012 box:

CREATE TABLE rdtest (label varchar(100), value int);

INSERT INTO rdtest VALUES
       ('one', 1)
      ,('two', 4)
      ,('three', 9)
      ,('four', 16)
      ,('five', 25)
;

ALTER TABLE rdtest ADD test_id int IDENTITY(1,1);

SELECT * FROM rdtest;


Result:

label    value       test_id
-------- ----------- -----------
one      1           1
two      4           2
three    9           3
four     16          4
five     25          5


Next:

ALTER TABLE rdtest DROP COLUMN test_id;

ALTER TABLE rdtest ALTER COLUMN value int NOT NULL;
GO

ALTER TABLE rdtest ADD CONSTRAINT PK_rdtest PRIMARY KEY ([value] DESC);

ALTER TABLE rdtest ADD test_id int IDENTITY(1,1);

SELECT * FROM rdtest;


Results:

label    value       test_id
-------- ----------- -----------
five     25          1
four     16          2
three    9           3
two      4           4
one      1           5


And, one more test run:

ALTER TABLE rdtest DROP CONSTRAINT PK_rdtest;

ALTER TABLE rdtest DROP COLUMN test_id;

ALTER TABLE rdtest ALTER COLUMN label varchar(100) NOT NULL;
GO

ALTER TABLE rdtest ADD CONSTRAINT PK_rdtest PRIMARY KEY ([label] ASC);

ALTER TABLE rdtest ADD test_id int IDENTITY(1,1);

SELECT * FROM rdtest;


Results:

label    value       test_id
-------- ----------- -----------
five     25          1
four     16          2
one      1           3
three    9           4
two      4           5


I suppose this isn't a guarantee that the clustered index dictates the order a new IDENTITY column gets filled in - but, I'd say it strongly indicates that it's very likely.