You will be able to use your 2009 families in 2010, but any family you create (or save) in 2010 will not work in 2009 or other previous versions. No backwards compatibility. I typically keep previous version families archived in case I need them. We currently have projects in 2008 and 2009 formats so I need the ability to load version specific families. For the time being I plan to keep creating families in 2009 until we start using 2010 on the majority of our projects.