Why do sequential GUID keys perform faster than sequential INT keys in my test case?
I modified @Phil Sandler's code to remove the effect of calling GETDATE() (there may be hardware effects/interrupts involved??), and made rows the same length. 

[There have been several articles since SQL Server 2000 relating to timing issues and high-resolution timers, so I wanted to minimise that effect.]

In simple recovery model with data and log file both sized way over what is required, here are the timings (in seconds): (Updated with new results based on exact code below)

       Identity(s)  Guid(s)
       ---------    -----
       2.876        4.060    
       2.570        4.116    
       2.513        3.786   
       2.517        4.173    
       2.410        3.610    
       2.566        3.726
       2.376        3.740
       2.333        3.833
       2.416        3.700
       2.413        3.603
       2.910        4.126
       2.403        3.973
       2.423        3.653
    -----------------------
Avg    2.650        3.857
StdDev 0.227        0.204


The code used:

SET NOCOUNT ON

CREATE TABLE TestGuid2 (Id UNIQUEIDENTIFIER NOT NULL DEFAULT NEWSEQUENTIALID() PRIMARY KEY,
SomeDate DATETIME, batchNumber BIGINT, FILLER CHAR(88))

CREATE TABLE TestInt (Id Int NOT NULL identity(1,1) PRIMARY KEY,
SomeDate DATETIME, batchNumber BIGINT, FILLER CHAR(100))

DECLARE @Numrows INT = 1000000

CREATE TABLE #temp (Id int NOT NULL Identity(1,1) PRIMARY KEY, rowNum int, adate datetime)

DECLARE @LocalCounter INT = 0

--put rows into temp table
WHILE (@LocalCounter &lt; @NumRows)
BEGIN
    INSERT INTO #temp(rowNum, adate) VALUES (@LocalCounter, GETDATE())
    SET @LocalCounter += 1
END

--Do inserts using GUIDs
DECLARE @GUIDTimeStart DateTime = GETDATE()
INSERT INTO TestGuid2 (SomeDate, batchNumber) 
SELECT adate, rowNum FROM #temp
DECLARE @GUIDTimeEnd  DateTime = GETDATE()

--Do inserts using IDENTITY
DECLARE @IdTimeStart DateTime = GETDATE()
INSERT INTO TestInt (SomeDate, batchNumber) 
SELECT adate, rowNum FROM #temp
DECLARE @IdTimeEnd DateTime = GETDATE()

SELECT DATEDIFF(ms, @IdTimeStart, @IdTimeEnd) AS IdTime, DATEDIFF(ms, @GUIDTimeStart, @GUIDTimeEnd) AS GuidTime

DROP TABLE TestGuid2
DROP TABLE TestInt
DROP TABLE #temp
GO


After reading @Martin's investigation, I re-ran with the suggested TOP(@num) in both cases, i.e.

...
--Do inserts using GUIDs
DECLARE @num INT = 2147483647; 
DECLARE @GUIDTimeStart DATETIME = GETDATE(); 
INSERT INTO TestGuid2 (SomeDate, batchNumber) 
SELECT TOP(@num) adate, rowNum FROM #temp; 
DECLARE @GUIDTimeEnd DATETIME = GETDATE();

--Do inserts using IDENTITY
DECLARE @IdTimeStart DateTime = GETDATE()
INSERT INTO TestInt (SomeDate, batchNumber) 
SELECT TOP(@num) adate, rowNum FROM #temp;
DECLARE @IdTimeEnd DateTime = GETDATE()
...


and here are the timing results:

       Identity(s)  Guid(s)
       ---------    -----
       2.436        2.656
       2.940        2.716
       2.506        2.633
       2.380        2.643
       2.476        2.656
       2.846        2.670
       2.940        2.913
       2.453        2.653
       2.446        2.616
       2.986        2.683
       2.406        2.640
       2.460        2.650
       2.416        2.720

    -----------------------
Avg    2.426        2.688
StdDev 0.010        0.032


I wasn't able to get the actual execution plan, as the query never returned! It seems a bug is likely. (Running Microsoft SQL Server 2008 R2 (RTM) - 10.50.1600.1 (X64))