I haven't received a file from someone who's jumped up to Microsoft Office 2007 that I couldn't read with Office 2003..... yet. Knowing the file formats are so different I have been expecting it to happen at some point. I just spotted this Office 2007 compatibility pack which says that it will do the job:
"When you install the compatibility pack, you can open, edit, save, and create files in the robust Office Open XML Formats."
Sounds good to me. I've installed it and it's added the 2007 and 2007 with macros formats to the save options. You can find it here.