For those people who like to use the Activity Monitor in the SQL Server Management Studio GUI instead of, or along with, sp_who, sp_who2 and DMVs in SQL Server, it is not where you left it in SQL Server 2005.
To view the activity monitor interface in SQL Server 2008 Management studio you need to:
1. Connect to the instance in Object Explorer
2. Right-Click the <Server Name>
3. Select <Activity Monitor>
It should the open as a pane on the right-hand window
Brad McGehee has a good article over on Simple Talk where he explains the new improvements and features that can be found in Activity Monitor in SQL Server 2008 http://www.simple-talk.com/sql/learn-sql-server/management-studio-improvements-in-sql-server-2008/