Came here to post this. This is very important and as a bonus, if you add some padding to the label the click area around the actual radio button becomes a bit larger.

You can combine nesting with for="id" to make sure you get complete compatibility.

I'm not sure about more recent versions of Safari, but I know this used to be the only way to get safari react to clicks on the label text.

I always remembered this as being "invalid" HTML, but a quick Google search turns up nothing. Can anybody shed some light?

It's not invalid; it's one of the two ways of using a label element. However, IE 6 didn't support it which is why I always resorted to the ID variant. The difference in typing isn't that large anyway.

Are you thinking Improperly nesting elements?

    <i>Some <b>text</i> here</b>

In that case, the label tag surrounding the input tag doesn't violate this rule.