Update, clarification: Current and past Microsoft Office versions, dating back to Office 2000 SR1, have a reduced functionality mode, as well. It is less forgiving this time around: While Office 2003 allowed users 50 "grace launches" without activation before shifting into reduced functionality mode, the 2007 Office programs will reduce the grace period to 25 launches.

The reduced functionality mode in the 2007 Office programs differs from Windows Vista by applying only to the initial retail product activation process. Windows Vista goes further by reducing functionality if the software doesn't pass an additional "validation" process. Not passing that extra validation step in Office prevents access to downloads and add-ons but doesn't reduce the software's basic functionality. I've changed the original post to reflect all of this:

Original post: Mary Jo Foley points to a Microsoft Office 2007 support document that describes how the productivity software will limit the functionality of copies that don't pass the company's activation process. It's part of Microsoft's broader effort to crack down on counterfeiting and illegal use of its software.

Although this doesn't make the program entirely unusable, it takes away key capabilities, prompting comparisons to a "kill switch." From the support page:

In reduced-functionality mode, programs function similarly to viewers. When a program runs in reduced-functionality mode, many commands are unavailable (dimmed). Therefore, you cannot access those functionalities. Some of the limitations of reduced-functionality mode include the following:

You cannot create new documents.

You can view existing documents. However, you cannot edit them.

You can print documents. However, you cannot save them.

Some Linux vendors saw Microsoft's anti-piracy approach in Windows Vista as a potential boon for their business. It will be interesting to see if the spotlight on Microsoft's anti-piracy initiatives also causes more people to consider OpenOffice.