Hey Soporifix,
In an attempt to make these forums actually helpful, here's a link to an article written on Time Magazine's Tech Section of their Internet site. It's written by the managing tech editor. I would imagine this guy possesses at least a modicum of technically savvy, or at the very least, has access to people who do. In any rate, he's having the same problem with his 4s that other people are complaining about.
http://techland.time.com/2013/06/06/apples-ios-6-1-3-update-can-knock-out-wi-fi- bluetooth-and-cost-you-200-to-fix/
He's asked Apple to comment, as of yet I don't believe he's been given a response.
Many people experiencing this problem have achieved temporary fixes by taking a heat gun to the back of the phone, putting the phone in the freezer for 30 minutes, or having the WiFi chip removed and resoldered (check out YouTube for examples). Granted these solutions are ridiculous and likely only temporary, they do however point to the root of the problem.
The growing consensus is that 6.1.3 is having problems with heat generation, especially around the WiFi chip. This is affecting the soldering, causing degradation in WiFi performance eventually leading to the point where the WiFi button becomes completely grayed out. The reason this happens to some 4s's and not others is that not all 4s's have the same WiFi Module or maybe be soldered differently. Check out EmBee82's posts on the following thread for a good summation of the problem and the MacGyver fix.
https://discussions.apple.com/thread/4909703?start=240&tstart=0
At any rate I wouldn't bother quibbling with anyone who might suggest the problem is on your end. As you and numerous others have mentioned, everything worked fine on the aggrieved devices until you updated to iOS 6.1.3. The only thing you've done wrong is to trust the efficacy of the latest iOS update. Keep pressuring Apple. They created the problem and the onus is on them to fix it.