Office 2003 and Office 2007
Things to consider if you MUST install Office 2003 and 2007+
Office 2003 and Office 2007+ use different versions of Visual Basic. At some point there is a likely possibility that you will experience a problem as a result of having the two installations.
Office 2003 and 2007+ tend to fight for ownership of files, Word 2007 one day then Word 2003 the next. The end-user will have to open the desired application and browse to the file. In a tandem installation the days of clicking a file and expecting Office to open with the intended version are gone!
Also at some point larger issues develop. I haven't been able to pinpoint the cause but it is likely caused when installing Visio 2007 or Project 2007. If the end-user is not careful and installs to default locations certain Office files get overwritten. All additional installations become custom and the file destinations need to be different.