Yes, it IS affecting 7% of non-jailbroken iPhones.
Go through geohot's Twitter history from approximately mid-November through mid December. He's spoken numerous times about the greyed out Wi•Fi bug.
It's a 3.1.2 compatibility issue, and it affects approximately 7% of ALL iPhones, jailbroken or not.
He even offered Apple a deal - if they paid him $10,000, he would fix the bug FOR them. He may have been joking, but they nver paid, so he never did it.
As far as this bug just randomly "occurring", there are apparently FOUR different versions of this bug. Only one of them is exaggerrated by blacksn0w, NOT blackra1n.
If you were going to have issues with Wi•Fi, you most likely would have had them as soon as you upgraded to 3.1.2. iPhones that were shipped with 3.1.2 will most likely not be affected. It seems to only apply to iPhones that were upgraded to the current OS.
I hope this answers some questions.
And in regards to the one shot jailbreaking thing, it DOES permenantly modify some files to allow future jailbreaking, but you're thinking of the foot-in-the-door method, a/k/a "PwnageTool". As long as you've been jailbroken before, PwnageTool will ALWAYS allow you to jailbreak AGAIN in the future. But stand-alone software, such as redsn0w or blackra1n wouldn't be able to do it unless they found a different exploit. The foot-in-the-door method is extremely aggressive. Newer exploits are always better.