Remember that, like it was in 2000 to 2005, the RDL specification changes. You'll be upgrade to 2008, but you won't be able to round-trip to back to 2005. You'll probably need to keep both versions of BIDs around until you've completely migrated.

 The big ugly change is in the Rendering Object Model. If you've written your own data processing or rendering extensions, you will probably enjoy the experience. Again.

 Quite a bit more here: http://msdn.microsoft.com/en-us/library/bb522762(SQL.100).aspx