SQL Server is a timeless data storage, however time travel is possible one-way only.
Unless you find a backdoor, of course.
What I mean by this is, that if you have a database residing on SQL Server 2000 and you back it up and restore it to a SQL Server 2008, then you will never be able to move the database back to SQL Server 2000, nor to SQL Server 2005.
Here is your backdoor, though: if you would like to migrate your database to SQL Server 2005 from SQL Server 2008, you would have to create a database on the SQL Server 2005 and use the Import-Export wizard to move the tables and the data from the 2008 to the 2005. After that, you can use the Generate Scripts task to move the rest of the objects: procedures, views, users and so on.
Fun, right? I wonder how many angry MS Word or MS Excel users would there be if they were told that they would never be able to use their files in previous versions of the product.