Every project is different, and you will encounter numerous combinations of Microsoft software installed. Some large corporations we work with, are still using Office 2000 and Office XP. The majority of enterprise accounts are still using Office 2003 on Windows XP. (I cannot find any hard figures on this though).
Client platform: Office 2007
Server Platform: SharePoint Portal Server 2003
You might have a customer that moved to Office 2007 but who is still using SharePoint 2003. There might be a number of reasons for this. Your customer probably has a very customized SharePoint 2003 and have decided not to upgrade it because it the solution work fine.
Feature Office 2007 with SharePoint Portal Server 2003
Saving documents to SharePoint sites Yes
Editing documents stored to a SharePoint site No (Edition only works for the Office 2003 formats.
You cannot save a DOCX back to SharePoint 2003)
Check-out/in, version history Yes
Updating properties Yes(it works but not as elegant as on SharePoint 2007)
In most cases you will run into this scenario. Your customer decided to implement SharePoint 2007, but they will still be using Office 2003. Here are differences between Office 2003 and Office 2007 when working with data from SharePoint sites.
Application / Feature Office 2003 Office 2007
Saving and editing Office 2003 files from SharePoint sites Yes Yes
Start a workflow No Yes