Remote Desktop Connection 6.0 client update is available for download in the Microsoft Download Center.

Microsoft Knowledge Base Article 925876 contains the following summary:

This article discusses the Remote Desktop Connection 6.0 client update that helps you use the new Terminal Services features. These features are introduced in Windows Vista and in the Microsoft Windows Server Code Name “Longhorn" operating system from a computer that is either running Microsoft Windows Server 2003 with Service Pack 1 (SP1) or Microsoft Windows XP with Service Pack 2 (SP2).

Microsoft Knowledge Base Article 925876 contains the following summary:

This article discusses the Remote Desktop Connection 6.0 client update that helps you use the new Terminal Services features. These features are introduced in Windows Vista and in the Microsoft Windows Server Code Name “Longhorn" operating system from a computer that is either running Microsoft Windows Server 2003 with Service Pack 1 (SP1) or Microsoft Windows XP with Service Pack 2 (SP2).