LINQ will be fully integrated with the next release of Visual Studio (code-name: Orcas) and it will include some very cool framework and tool support (including full intellisense and designer support). Last week the LINQ team released the May CTP drop of LINQ that you can download from here. What is cool about this CTP is that it works with VS 2005, and allows you to start learning more about it immediately. It incorporates a bunch of customer feedback (for example: support for stored procedures in DLINQ), and also includes a built-in ASP.NET Web-Site Project to enable you to leverage it with ASP.NET apps (note: you can also use LINQ with the new VS 2005 Web Application Project option as well).
While they're not yet complete, applications written for the standard Microsoft .NET framework can now be ported and run on Linux through Mono. However, holes within the framework implementation on Mono mean you'll still need to check that your application doesn't use any services that are not yet implemented.