Illustrator 9 contains the old Pantone libraries, and Illustrator 10 contains the new Pantone libraries.
Chelan
I would presume that Pantone's new definitions are more accurate given that they are not limited to the percentages of the old "letter tints". More accurate, that is, based on SWOP standard, which is really not so much of a standard as one might think.
:) Mordy