If you check Kalen Delaney's Inside SQL Server 2000, Chapter 7, Corrolated Subqueries, Page 354, she explains that using the LEFT JOIN and testing for null uses less than half the logical I/O's of the corrolated subquery.  Usually a join is more efficient than the alternative.

You may have indexes that could effect the outcome.

If you use alot of corrolated subqueries you may have to work on thinking more set based, and less row-by-row.

Jeff Lomax