No, since .NET 4 is not a requirement for CMS 6. It uses System.Web.Caching.Cache. I think however that it is possible to create your own implementation of the EPiServer.BaseLibrary.IRuntimeCache interface to change this behaviour.

Thanks! Based on the documentation you provided the provider seems to be customizable.

The class EPiServer.CacheManager has been expanded with a RuntimeCache property to allow developers to access an instance of the runtime cache. In addition, the following static helper functions have been added to this class

This blog post (which is actually about something else) by Ted shows how you can use the ClassFactory class to register your own implementation of the IRuntimeCache: http://tedgustaf.com/en/blog/2010/8/unit-test-an-episerver-6-website-using-visual-studio-2010-and-mstest/