Anyone who purchases Office 2010 from October 19th will be eligible for a free upgrade (costs for postage, if required) to Office 2013 when it is available in 2013.
Microsoft's Office News blog had this to say about Office 2013:
This is the most ambitious release of Office we've ever done. It spans the full family of Office applications, servers and cloud services. The new Office has a fresh, touch friendly design that works beautifully on Windows 8 and unlocks modern scenarios in social, reading, note-taking, meetings and communications. We are proud to achieve this milestone and are eager to deliver this exciting release to our customers.
The Outspoken Wookie