I have been working with SQL 2000 and since that knowing
that SQL server ill use and release memory depending on its requirement and unitization.
This is not true with SQL 2008 while it
is installed on Windows server 2008.
SQL 2008 will utilize as much memory assign to SQL Server
while high resource oriented task running. It will never releasing this memory
back to system though task finish. If any other task needed memory SQL
internally flush out the older data from the memory. To release memory reserved
by SQL 2008 you need to re-start SQL services.
I also observed sometime in task manager also assigned
memory not showing against the SQL Server.