Here's a function for doing dateadd for weekdays only.  (e.g., what weekday is 43 weekdays from today?)  Hope it is helpful.

CREATE FUNCTION dbo.weekday_dateadd ( @nbrOfDays int, @startDate smalldatetime )

RETURNS smalldatetime
AS
BEGIN

/*
  Title: weekday_dateadd
  Author: Jesse Roberts
  Date: 08/03/2005
  Purpose: Adds a specified number of weekdays to a given date and returns the resulting date
*/

declare @adj_start smalldatetime, @endDate smalldatetime
declare @adj_nbrOfDays real, @adj_dayofweek int
declare @numweeks int, @adj_enddays int, @calendar_days int

-- get to the last Monday prior to the start date
set @adj_dayofweek = (datepart(dw,@startDate) - 2)
set @adj_start = dateadd(d, -(@adj_dayofweek), @startDate)

-- round to the nearest multiple of 5
set @adj_nbrOfDays = round(@nbrOfDays / 5, 0) * 5

-- adjust the rounding product to ensure the result is the last multiple of 5 less than @increment
if @adj_nbrOfDays > @nbrOfDays
 begin
   set @adj_nbrOfDays = @adj_nbrOfDays - 5
 end

-- add up the number of calendar days to add back to the end date
set @adj_enddays = (@nbrOfDays - @adj_nbrOfDays) + @adj_dayofweek

if @adj_enddays > 4 -- if @adj_enddays is > 4, then it means Friday of the week was passed since 2 (Mon) + 4 = 6 (Fri)
 begin
   set @adj_enddays = @adj_enddays + 2 -- add 2 days to compensate for the Sat/Sun wrap
 end

set @calendar_days = (@adj_nbrOfDays * 1.4) + @adj_enddays

set @endDate = dateadd(d, @calendar_days, @adj_start)

set @endDate = case datepart(dw,@endDate)
     when 7 then dateadd(d,2,@endDate)
     when 1 then dateadd(d,2,@endDate)
     else @endDate
        end

RETURN @endDate
END