Is that I haven't seen it. In the office I currently manage users even open Corel Wordperfect along with Office 2003 docs using a mix of Office 2007 & Office 2010. On the few Office 2003 application specific installs the Office 2007 conversion kit works wonders for forwards compatibility.
My last horrible conversion experience was when users moved from Access 97 to Access 2003.
ADDNEDUM: Thought I would point out that later versions of Office apps even warn you about compatibility problems when you save to older formats. You get the option of saving to an upgraded format. Now that would break compatibility, but that aint a technical issue.
