My own (over?)simplified interpretation follows:

In past updates, it has been the usual practice to make newer versions automatically and by default able to import older Office files.

For some recent updates it has been necessary to separately install export filters to allow the new version to save files that could be read by an older one; and addition of import/export filters in the older version to allow it to use files from a newer one was fairly common.

For Office 2007, it was decided that the "readers" for some older files present a "security risk." The files themselves are not a risk – only the programs that "read" them are risky.

By default, Office 2007 may block access to Office files produced by older versions of Word, Excel, and Power Point. Files produced by some older versions of CorelDraw may also be blocked.

Office 2003, after SP3 is installed, will generally block access to the same files.

In Office 2007 it is claimed that you can "easily" create a "trusted folder" and anything in that folder will be opened without checking with the new security system that would otherwise block them. (The new child-speak for the security system is "The Trust Center." Typically "cute" for Office 2007.)

The "trusted folder" method probably can be used in Office 2003 SP3, but is reportedly "more difficult." I haven't checked out "how much more difficult."

For Office 2003 SP3, the recommended method of accessing the files is to make a series of Registry Edits. Instructions given in applicable Microsoft KB articles are – admitted by LeBlanc – "vague and confusing."

Updates to a couple of KB articles, to clarify them and to provide "automated" methods for turning access to the files in question on or off are promised. A quick look indicates that not all the updates promised have been made. The Office 2003 SP3 "fixes" will consist of a set of 8 .reg files that you can download and keep handy. Four of these will turn on access, each to a different kind of file, and the other four will be for turning off access when it's no longer needed.

For those not familiar with them, a .reg file is a "registry archive" file that saves all or a portion of the registry. If you "double-click" a .reg file, it's contents are instantly written into the Registry.

Anyone who has Office files going back to Win95 and possibly to Win98 and who may someday have Office 2003 or later, may someday wish they had a bookmark to "how to open the old ones."

The above linked blog entry has the necessary info; but for a more "stable" source I would recommend noting:

KB 922849 Describes how to create a "trusted folder" from which the legacy files can be opened. (Easiest in Office 2007)

KB 938810 Description of the format support changes.

After noting the locations, check back according to your own inclination, to see if the promised "clarifications" appear.

(Let us all now emit a resounding roar signifying our unanimous approval for progress.)

John