Just quickly checking Yahoo! last 10Q:
QUOTE(Balance sheet)
Common stock, $0.001 par value; 5,000,000 shares authorized; 1,497,912 and 1,525,277 shares issued, respectively, and 1,360,247 and 1,330,063 shares outstanding, respectively
Treasury stock at cost, 137,665 and 195,214 shares, respectively
10QOut of the 3 million shares outstanding, Yahoo! only has 3 hundred thousand shares of treasury stock. Assuming the board directors cannot make up the other 40%, it looks like Microsoft will have more than enough wiggle room to pull it off.