|go ahead... be a heretic
Just a note that index implementations vary widely from platform to platform. As others have pointed out above, PG and Oracle (among others) implement function indexes. Some others won't use an index on that query at all because of the less-than condition.
The key thing to note is that indexes are not "magic". Some programmers seem to believe that whenever a column is involved in a query, adding an index will make the query faster. This is rarely true. As in optimizing your Perl code, you should measure, measure, and measure again, then make a choice based on that.