I would do this:

public static IEnumerable<DateTime> GetIntervals(DateTime clockIn, DateTime clockOut)
{
    yield return clockIn;

    clockIn = clockIn.AddHours(1).Subtract(clockIn.TimeOfDay).AddHours(clockIn.Hour);

    for (DateTime dt = clockIn; dt < clockOut; dt = dt.AddHours(1))
        yield return dt;

    yield return clockOut;
}

Use it like this:

    foreach (DateTime dt in GetIntervals(DateTime.Parse("5/25/2011 1:40:56PM", CultureInfo.InvariantCulture), DateTime.Parse("5/25/2011 6:22:12PM", CultureInfo.InvariantCulture)))
    {
        Console.WriteLine(dt);
    }