You can't index the expression used in this example, because it's not deterministic (includes GETDATE function). However, if what you really need is to query Orders created in the past X days, then this can be facilitated with an index keyed on OrderDate.
Another little known fact is that later editions of SQL Server now support creating indexes on non-persisted computed columns.
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho