select * from [Knowledge_Base]
  where Date_Opened <= '2010-09-30'
  and Closed Is NULL or Closed > '2010-09-30'

create table #Month 
(
	MonthStartDate		datetime not null,
	NextMonthStartDate		datetime not null primary key clustered
)

-- Create table of month start dates
insert into #Month
select 
	MonthStartDate	= dateadd(mm,datediff(mm,0,getdate())+a.number,0),
	NextMonthStartDate	= dateadd(mm,datediff(mm,0,getdate())+a.number+1,0)
from
	-- Number Table Function available here
	-- http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=47685
	http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=47685
	dbo.F_TABLE_NUMBER_RANGE(-12,0) a
order by
	a.number
	

select
	b.MonthStartDate,
	a.*
from
	[Knowledge_Base] a
	join
	#t b
	on
	a.Date_Opened	<  b.NextMonthStartDate and
	(a.Closed	>= b.NextMonthStartDate or a.Closed is null )
order by
	b.MonthStartDate