I had that question today as I just branched a piece of my current project into its own side project on CodePlex. I wondered since I was using TFS as the default source control plug-in in VS.NET whether I couldn't open my SVN-controlled solution. It turns out, you can! Apparently it is solution-scope and not application-scope, so in one VS instance you could have a TFS project open, in the other, your SVN project. That is my current state right now.

If you're wondering, the project is jForms.NET, hosted on CodePlex. I am moving my current code into the new C# project, improving it to support better flexibility for other people's needs. I need to finish it so I can subclass it for my current project to support custom Linq to SQL validation (i.e. no validation controls on forms!), which I will also release later too.