[rjvin]

I don't think the application of this technology would be the way it is on this page: likely it would detect your face and then set the text accordingly, and then leave it alone. A constantly changing text size is bad interaction anyway.

[saalweachter]

It also shouldn't be in websites. This would make an awesome browser plug-in, which would also let you calibrate it (as mentioned by someone else) to be the "right size" for your vision.

I think a U-shaped curve could also be good: big text when you're far away, small text when you're at normal viewing distances, and zoom further [esp for full-page zoom] when you lean in very close.