Personal hotspot works perfectly. Actually, even better than perfectly since it's activated on my iphone without me paying anything extra for it. Very nice.
It looks like it doesn't work on 3GS. I wonder if there's any technical limitation preventing it to work or just a marketing strategy.
As a jailbreak developer: there is not.
Doesn't it? It is present on mine, but when I click to set it up, it tells me to contact my carrier to enable it on my account, so I haven't tested it.