One solution for Office 2007 users who want to ensure that their documents can be read by recipients is to make the documents backwardly compatible by save them as, well, "backwardly compatible".
I just noticed a pretty disturbing post on Digg. According to Tim Gaden, Microsoft Office 2007 saves files in a new xml-based file format by default. The problem with this is that users of Microsoft Office for the Mac will not be able to open these files. The file formats typically end with an ‘x’ so Word documents have the file extension ‘.docx’, Excel documents have the file extension ‘.xlsx’ and PowerPoint files have the extension ‘.pptx’ and so on. Although there is apparently a compatibility pack for Office 2003 users, no such thing exists for the Mac version just yet. It is probably just a matter of time until an update is made available but for the time being, watch out for those so-called "Microsoft Open XML Formats". If you are curious what an xml document may look like in Word for the Mac, here is an idea: