See What Other Users Have Checked Out in TFS

I found the answer on Adam Prescott's blog

Prerequisites

Visual Studio Team Foundation Power Tools – With Visual Studio 2010 – in the Tools menu, select Extension manager. Download the Power tools and install (close VS 2010)

See what others have checked out

After the installation of the tools has finished:

• run VS 2010
• open the Source control explorer window
• In the VS 2010 main menu, select File / Source control / Find In Source Control / Status…
  
• In the dialog box, leave the path to $/ to find checked out sources in all workspaces, click on ‘find’
  
• You’ll be presented a list of files with the user names that are checked out