Is Oracle 10g smart enough to work on a WHERE clause identically as the equivalent INNER JOIN?
Is it possible?  Sure.  Is there likely to be an improvement in performance?  No.  If there is a change in performance (barring cases where you discover that a join is missing or otherwise fix a query), it'm more likely that the old implicit join syntax will be more efficient.  But that's pretty unlikely.

Behind the scenes, when you have a query using the SQL 99 inner join\'left outer join` syntax, Oracle actually transforms that to use the old style syntax before optimizing it.  When Oracle first introduced the SQL 99 syntax, there were occasionally bugs in this transformation process.  By the time you get to 10.2.0.4, the odds that you'd encounter one of those bugs is pretty low.  But if you look at enough queries, you might find one that is more efficient using the old syntax.