You'll just have to be careful when you import any ongoing existing Word/Excel documents into Google Docs. It's only the importing process that will lose that info. After that, Google Docs/Google Spreadsheets will track any changes that are made within it.
Yeah, and Google Docs helpfully removes change tracking from documents created in OpenOffice.org or Symphony and saved in ODF (any version), too. I'm looking forward to the flood of complaints about IBM and Oracle writing change-tracking metadata that Google Docs loses on import.