This standalone application upgrades ASP.NET MVC 2 applications to ASP.NET MVC 3. It works for both ASP.NET MVC 3 RC 2 and RTM. The tool only supports Visual Studio 2010 solutions and MVC 2 projects targeting .NET 4.
It will not work with VS 2008 solutions, MVC 1 projects, or projects targeting .NET 3.5. Those projects will first have to be upgraded using Visual Studio 2010 and/or retargeted for .NET 4. The tool will:

• Create a backup of your entire solution
• Update all Web Applications and Test projects to reference System.Web.Mvc v3.0
• Add references to System.Web.Helpers.dll and System.Web.WebPages.dll (new libraries required by MVC 3)
• Upgrade web.config files to reference System.Web.Mvc v3.0 and add configuration and settings to support the new Razor view engine
• Add or upgrade JavaScript files that ship with ASP.NET MVC 3 (jQuery 1.4.4, jQuery UI 1.8.7 jQuery Validation 1.7, Microsoft AJAX)
• Add stylesheets and image assets to support jQuery UI

More information is available on Marcin's blog.
This tool is an updated version of what was previously released via Eilon's blog.
Update on 4/11/2011: Added option to skip performing a backup