Microsoft owns the "Ribbon" look-and-feel
Although the license is free, there is a catch: if you accept the license, you also accept many "must-follow" guidelines which ensure consistent implementation of the Ribbon across the globe. Another somewhat more contraversial catch is that you cannot license Ribbon for competing software, on any platform:
The license excludes products or components that perform primarily as software for word processing, spreadsheets, presentations, email, contacts and databases, and that are created or marketed as a replacement for any or all of them.
This may seem reasonable from Microsoft's perspective: they've spent years testing and developing the Ribbon UI concept and they don't want to just hand it to someone else to compete with them. Unfortunately, this leads to a slippery slope of owning UI concepts, which are never so black and white. For example, the Ribbon borrows heavily from the concept of toolbars; many people have used toolbars in a number of configurations (including Microsoft themselves). What makes a toolbar no longer a toolbar but a license-infringing Ribbon?
This move also ensures that software like OpenOffice or Thunderbird will have to use different interface to Office 2007. This will place a greater support burden on companies that decide to switch to alternatives.
Moreover, historically Microsoft's Office UI has been copied over and over again, giving competing applications consistent look and feel (Microsoft is not alone in this, just ask Lotus and Adobe). Even webmail providers have emulated Outlook's interface to make it easier for their users to get used to the new system. Microsoft has not complained about this sort of thing in the past as it benefited indirectly: wide usage of their UI concepts validated the Office suite and ensured it was the "gold standard" of usability and the safe choice. This also helped standardize the Windows GUI across variety of applications. The new licensing arrangement sets a bad precedent for application development not just on Windows, but on any platform.