A bunch of round-ups on the issue, mainly concerning performance (isnull is faster in most of the tests, but then there's a lot of debate about what is being tested):
http://dotnetjunkies.com/WebLog/ekepes/articles/13938.aspx
http://www.aspfaq.com/show.asp?id=2532
http://blogs.x2line.com/al/archive/2004/03/01/189.aspx
http://sqljunkies.com/WebLog/amachanic/archive/2004/11/30/5311.aspx
http://weblogs.sqlteam.com/mladenp/articles/2937.aspx
http://toponewithties.blogspot.com/2004/08/differences-between-coalesce-and.html
http://blogs.x2line.com/al/archive/2004/03/01/189.aspx