INSERT INTO a temp table, and have an IDENTITY field created, without first declaring the temp table?

Oh ye of little faith:

SELECT *, IDENTITY( int ) AS idcol
  INTO #newtable
  FROM oldtable

http://msdn.microsoft.com/en-us/library/aa933208(SQL.80).aspx

You commented: not working if oldtable has an identity column.

I think that's your answer. The #newtable gets an identity column from the oldtable automatically. Run the next statements:

create table oldtable (id int not null identity(1,1), v varchar(10) )

select * into #newtable from oldtable

use tempdb
GO
sp_help #newtable

It shows you that #newtable does have the identity column.

If you don't want the identity column, try this at creation of #newtable:

select id + 1 - 1 as nid, v, IDENTITY( int ) as id into #newtable
     from oldtable

Good Question & Matt's was a good answer. To expand on the syntax a little if the oldtable has an identity a user could run the following:

SELECT col1, col2, IDENTITY( int ) AS idcol
INTO #newtable
FROM oldtable

That would be if the oldtable was scripted something as such:

CREATE TABLE [dbo].[oldtable]
(
    [oldtableID] [numeric](18, 0) IDENTITY(1,1) NOT NULL,
    [col1] [nvarchar](50) NULL,
    [col2] [numeric](18, 0) NULL,
)