This could block Linux from booting, but realistically speaking, does anyone believe that will happen? It seems very, very unlikely to me that you won't be able to disable signing restrictions at the firmware level.
Investors are savvier than you'd think. Even on a rumor that the vendor is doing an exclusive lock-in with MS, I'm sure you'd see the stock price dip. Investors spend all their time looking at news reports in their target industry, so I'm sure they'll notice something as big as this.
But what product will they be putting the lock-in on? It may not matter, as we've seen with mobile OSes which, from the perspective of the median buyer, are locked in.
It makes somewhat some sense that MS will do this especially since Hyper-V (VM Hypervisor) is now built-in to Windows8. "Want Linux? You need Windows too..."