Short patent timeline:
16 May, 1991: "If people had understood how patents would be granted when most of today's ideas were invented, and had taken out patents, the industry would be at a complete standstill today." - Bill Gates, Challenges and Strategy Memo
Aug, 2004: OSDL releases a study saying Linux may infringe 283 patents; Ballmer leaves away the word "may"
May, 2007: Brad Smith claims Linux potentially infringes 235 patents
Oct, 2007: Ballmer claims
Linux potentially infringes 153 patents
Conclusion: Number of Microsoft patents filed at USPTO grows, but number of MS patents Linux potentially infringes on is declining.