Never had a good reason to do this up until now, but if you need to get a value from a generated sql string, there are a few hoops you must jump through. Example:
declare @table varchar(20)
--can't use double quotes w/out this :)
SET QUOTED_IDENTIFIER OFF
--set your variables
select @table = 'tblcustomers'
,@username = 'webmaster'
--create a temp table to hold your value
create table #temp (field varchar(50))
--insert the value into temp table
insert into #temp
exec ('select email from ' + @table + ' where username = "' + @username + '"')
select field from #temp
drop table #temp
Reference: Getting Creative with EXEC and EXEC()