The current release of Universe Sandbox requires .NET 2.0 or higher.
The current release of .NET is 3.5.
Universe Sandbox will run the exact same way on a computer with .NET 2.0 as it will on a computer with .NET 3.5.
When I release a version of Universe Sandbox with multi-core support everyone will have to upgrade to .NET 4.0 in order to run that version of Universe Sandbox.
Even if .NET 4.0 was already out, nothing in Universe Sandbox would change until I release a new version that takes advantage of the new capabilities in .NET 4.0.
Does that make sense?