Cringely writes in his weekly I, Cringely that Microsoft getting into the anti-malware business is a bad idea, both from their motivations and history:
Microsoft has always hated firms that sell products that enhance their operating systems. They hate sharing revenue with others. Microsoft has to be envious and annoyed by the fact Symantec and others get more recurring revenue from Windows than Microsoft does.
Time will tell.
I don’t think Microsoft will be offering a version for OS X or Linux.