Except then, if you're trying to have it work one way on a device and you navigate to it via an on-site link, you have to "flip" modes. If it's an option, I'd like it to be a tri-state - "Never" / "Non-Mobile Only" / "Always" - so if the site detected a mobile device, it could skip the updating.

I'd LOVE this on my computer. I wouldn't hate it on my phone (I have unlimited bandwidth), but it'd be a silly use of resources since I can't see page titles there unless I'm actively flipping between them. Especially if I wasn't even in the browser at the time but had just left the page up, although I admit that'd be more likely if my phone browser were better (since I have 50/50 odds it'll refresh the page when I go back in anyway, leaving a reading page up to track where I was is fairly pointless).