--Your Sample data
declare @t table (UserID varchar(10), IncidentNo varchar(10), DateTime [datetime])
insert @t 
select 'User1', 'XPI1243', '12/8/2008 9:00:01 PM' union all
select 'User2', 'XPI1243', '12/8/2008 9:45:01 PM' union all
select 'User2', 'XPI1243', '12/8/2008 10:20:35 PM' union all
select 'User1', 'XPI1243', '12/8/2008 11:55:00 PM' union all
select 'User1', 'XPI1243', '12/9/2008 1:12:10 AM' union all
select 'User2', 'XPI1243', '12/9/2008 5:05:41 AM' union all
select 'User1', 'XPI1243', '12/9/2008 5:55:01 AM' union all
select 'User1', 'XPI1243', '12/9/2008 9:00:01 PM' union all
select 'User2', 'XPI1243', '12/9/2008 9:45:01 PM' union all
select 'User2', 'XPI1243', '12/9/2008 10:20:35 PM' union all
select 'User1', 'XPI1243', '12/9/2008 11:55:00 PM' union all
select 'User1', 'XPI1243', '12/10/2008 1:12:10 AM' union all
select 'User2', 'XPI1243', '12/10/2008 5:05:41 AM' union all
select 'User1', 'XPI1243', '12/10/2008 5:55:01 AM'

select * from @t where userid = 'user1' order by [datetime]

;with ShiftGroup (grp, startShift, endShift)
as ( --Create a table with your Shifts for each day
select v.number grp
       ,dateadd(hour, 21, dateadd(day, number, mn)) startShift
       ,dateadd(hour, 6, dateadd(day, number+1, mn)) endShift
from   (
       select datediff(day, mn, mx) days
              ,mn
              ,mx
       from   (
              select dateadd(day, datediff(day, 0, min([datetime])), 0) mn
                     ,dateadd(day, datediff(day, 0, max([datetime])), 0) mx
              from @t
              ) d
       ) d
inner join master..spt_values v
       on v.type = 'P'
       and v.number <= days
)

--Final Select GROUPed by Shift
select t.UserID
       ,sg.startShift
       ,sg.endShift
       ,count(*) [requestCount]
from   ShiftGroup sg
inner join @t t 
       on t.[datetime] >= sg.startShift
       and t.[datetime] <= sg.endshift
group by t.UserID
       ,sg.startShift
       ,sg.endShift



OUTPUT:
UserID     startShift              endShift                requestCount
---------- ----------------------- ----------------------- ------------
User1      2008-12-08 21:00:00.000 2008-12-09 06:00:00.000 4
User1      2008-12-09 21:00:00.000 2008-12-10 06:00:00.000 4
User2      2008-12-08 21:00:00.000 2008-12-09 06:00:00.000 3
User2      2008-12-09 21:00:00.000 2008-12-10 06:00:00.000 3