After producing some documents in Word 2008, however, I grew to like some of the improvements, such as the improved way of working with header and footers.
Recently, however, I was helping a student with his 670-page Master's thesis, which was produced in Word 2003. I couldn't open (or thought I couldn't) his file in Word 2008. My next-cube-neighbor couldn't either, but he could open it using Office 2008 on his Mac laptop. So, he created a 2008-combatible (i.e., docx) file for me, and I got the same results. It really turned out that the file could load -- it just took 17 minutes to actually open!
In an experiment, I used VMware to open another XP session within my current one. I have Office 2003 installed under that session. Despite all of the inherent disadvantages of running Office under a virtual machine, Word 2003 was able to open it almost instantly.
Making a long story short, I think that Word 2008 had a real problem with the styles that the student was using. Most of his styles were variations of the normal tag, and I've read on Woody's that this type of "direct formatting in very large documents usually causes corruption."