Microsoft announced a program last month that allows you to purchase Office 2007 (full version, not upgrade) and be eligible to download and register Office 2010 when it is available this June for Free. Details are at the Microsoft Technology Guarantee page:

Office 2010 Technology Guarantee – is the following:
• Purchase Office 2007, or a new PC with Office 2007, and activate it between March 5, 2010 and September 30, 2010.
• Have, or create a Windows Live ID.
• Redeem your Tech Guarantee before October 31, 2010.
• The Office 2010 Technology Guarantee will be fulfilled online, via download, at no additional cost.

– Troy @ TLC