Apple acknowledged problems with the Leopard upgrade as per this cNet/News.com article:

http://www.news.com/8301-13579_3-9806005-37.html

Hopefully, it's not as bad as the XP to Vista upgrade. I've been holding back on upgrading to Vista after learning my Adobe CS2 suite which I purchased in 2004 for $600 is not compatible. Both my sons are running Vista on their desktops and neither are very happy with its performance at boot up or with games.