While I hadn't ever heard of Yammer, just one look at the home page makes it seem as though it's a Facebook rip-off (no other way to really say it) just for use in a corporate setting.
Seems as though he's just a little worried that if Facebook bows to the patents his company will be next on Yahoo's hit list.
And worse, the whole layout is a Facebook ripoff. My girlfriend uses Yammer for work and when I took a closer look at it today it seems that most of the IU elements are 1 to 1 copies of Facebook. Pretty sad to be honest.
Seems as though he's just a little worried that if Facebook bows to the patents his company will be next on Yahoo's hit list.