Absolutely. It's happened (and happening) to Android. Carriers block rooting. Carriers block sideloading. Carriers add apps that can't be removed. They swap search engines and remove the settings. Each time a change flies in the marketplace, they get emboldened.
Today, experiencing the "choice" Android promised is contingent on flawless and thorough research, with tomorrow looking increasingly worse.
But where's the earnest geek concern about the slippery slope when we talk about Android fragmentation? That ball is busily picking up speed, but we don't talk about it.
We talk about how fragmentation isn't real or isn't a big deal. Even while every tom, dick and harry lets loose their plans of an Android future that doesn't include Google's marketplace, likely won't include side-loading, rooting or half the "choice" we like to pretend separates Android from iOS.
Yet in the context of Apple --where this change would require Apple reversing momentum, against developer wishes, after having quite publicly reversed the scope-creep of 3.3.1 after developer backlash-- we seem to be unable to talk about anything else.
So while an Apple U-turn is a legitimate concern, it's still FUD to put this big a spotlight onto it for those two big reasons:
1. Recent evidence suggests, if anything, it's less-likely than ever.
2. The focus is completely out of proportion with what the issue receives in other contexts.
The more realistic outcome out of this is that the App Store will be so popular that it marginalizes any developer who uses any other method of distribution.
That, I see as a negative, and more likely outcome than a complete application lockdown on the OS.
It's also worth noting that the slippery slope argument is widely regarded as a fallacy: http://en.wikipedia.org/wiki/Slippery_slope
I know CS5 is now 64-bit on both Windows and OSX, but the OSX users had to wait an extra 18 months because of Apple, not because of Adobe.
With the phone, it was a new device, and in many ways, it makes sense to lock it down.
The Mac has a 25 year legacy, and if they were to take away that choice, so many people would leave the platform. Apple might be jerks a lot of the time, but they're hardly stupid.