Finding gaps (missing records) in database records using SQL

I have a table with records for every consecutive hour. Each hour has some value. I want a T-SQL query to retrieve the missing records (missing hours, the gaps). So for the DDL below, I should get a record for missing hour 04/01/2010 02:00 AM (assuming date range is between the first and last record). Using SQL Server 2005. Prefer a set based query.

DDL:
CREATE TABLE [Readings](
    [StartDate] [datetime] NOT NULL,
    [SomeValue] [int] NOT NULL
)
INSERT INTO [Readings]([StartDate], [SomeValue])
SELECT '20100401 00:00:00.000', 2 UNION ALL
SELECT '20100401 01:00:00.000', 3 UNION ALL
SELECT '20100401 03:00:00.000', 45

Copyright Notice：Content Author:「Tony_Henrich」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/2693171/finding-gaps-missing-records-in-database-records-using-sql