Version 9 can't access version 6 datasets directly, you have to use Proc Migrate to convert them.
Version 9 as Klaz says has a bunch of new functions included, my personal favourites are the CATX, and CATT functions.
There's also a nifty new ODS Tagset called EXCELXP which allows you to write output directly into an Excel Spreadsheet (sort of) and control the formatting, colours etc etc.
Migrating is relatively painless, we did it in a reasonable sized org with about 40 or so SAS Programmers in one evening. We actually installed version 9 a couple of months earlier and got a number of trusted users to run tests on the usual processes to make sure that everything was smooth. We found a couple of minor differences which affected output, specifically the way SAS handled NULL characters (ASCII 0) in data fields. Version 9 reads these as valid characters, whereas in version 8 these were considered to be empty fields, which caused a few variations in record counts etc. This shouldn't be an issue for most cases, however it can if you have some dodgy data (I did).
Chris
Business Analyst, Code Monkey, Data Wrangler.
SAS Guru.