I just finished looking at the Microsoft page... and it may be true... or it may be false. The page is written in a very turgid and unclear manner. I suggest anyone worried about it try and read the page themselves and see what they take from it.
http://support.microsoft.com/kb/938810
Well, I'm no fan of MS, as you know... but I can't believe that Microsoft would cut off the ability to read files created with office versions within the past couple of years. That's insane. They might do it by accident, but not intentionally. Older than a decade? Sure, though even that is, in the words of one of Microsoft's own people, "draconian".
One way or t'other I'll know soon -- my users who have Office 2003 have to get at old files created by Office 2000, Office XP, etc. If they're blocked, my work is cut out for me.