I don't see the changes today as a U-turn, but simply coming to realise that the previous deals were just too damn good (I've said this for ages, and people I know high up within Three have known and said this for ages too).
Ultimately, Three was the small player that needed to shake things up. And the rest of the competition just looked down and laughed at how Three was acting like a mug, giving everything for free and never looking like it would be able to make money.
And make money is something Three needs to do, given it's now building a 4G network.
So, clearly last year, Three realised it had to still try and compete (with things like Feel at Home, free calls to 0800 and AYCE data on the handset) but also stop giving away too much. Unlimited tethering once people were getting DC-HSDPA speeds was bad enough, for 4G it's potentially going to be a big problem one day (I don't think it's a big problem right now).
Three is simply doing what any good business should do and pre-empting any future problems, and also making sure that it doesn't give away too much. As long as it can still offer a tariff with AYCE data, a generous tethering amount (more than many tariffs offer as standard on other networks for anything close to the same amount of money) then it will still be the best value.
Three is ahead on 3G coverage, but behind on 4G and with no 2G fallback, so there are still other considerations, but if you have 3G or 4G then I think Three still has an unbeatable offering.
EE has brought down its 4G charges, or increased allowances, so we have to assume that Three has played a part in improving what the competition is offering too - so even if you don't sign with Three, simply having Three in the market is doing good for the consumer overall.