All of you who get the ambiguous "Unknown Error", "An Error has Occured" or the dreaded "The File is Not Readable" errors in Illustrator CS 4. I bring you bad news. Nothing in CS 5 fixes these issues.
The main problem is that for a percentage of Illustrator users, the 32 bit nature of the application limits the number of linked objects on a page. It seems that Adobe wants to force us to use InDesign for page layout, even though Illustrator better suits the specialized purposes of certain markets.
So, until Adobe pulls it's collective head out of neither regions and produces a 64 bit native version of Illustrator, look forward to more "The file is not readable" messages popping up as you try to open that critical, deadline-nearing, project.