Microsoft was sued back in 2007 by a company named i4i, whose XML editing patent was breached upon when MS put it in Office 2007's Word. Microsoft says it's a "little used feature", but it's that little feature that's going to cost them $290 million in damages, as well as force them to change up their software to abide to the court's ruling. 

"We couldn't be more pleased with the ruling from the appeals court which upheld the lower court's decision in its entirety. This is both a vindication for i4i and a war cry for talented inventors whose patents are infringed. The same guts and integrity that are needed to invent and go against the herd, are at the heart of success in patent litigation against a behemoth like Microsoft."

                                                                                   -Loudon Owen, i4i Chairman

Some people were starting to fear that Microsoft would have to start pulling down copies of Word and Office off of store shelves, but it seems that's not the case as the court ruled that only copies sold after Jan. 11th, 2010 would have to have the XML feature removed.

Also, just to note, Office 2010 does NOT have the technology that infringes on the XML patent at all.

[PC Mag]