Potentially breaking a large amount of website isn’t something to take easy. And you can’t say that “Their webmaster should take care of it.” cause many don’t have a permanent webmaster and also isn’t particularly willing to suddenly shell out for an update when the old site “just worked”.

This new IE8 opt-in could be good, IF it’s a one time opt in trigger. The opt in would mean that once opted in the site has opted in to ALWAYS render with the latest rendering engine.

In which case we’d then have three modes: (I’m ignoring the mode inbetween Quicks and Strict here)
Quirks — the HTML soup from the dark ages of doom
Strict — IE6/7’s attempt for standard but which still shows it scars of the old soup model
Standard — Up to date rendering in line with Mozilla, Safari and Opera continually maintained which hopefully Microsoft will throw their own IE into.

The simple painful fact of a webdeveloper’s life is that support is that we just can’t suddenly drop support for the browser with the biggest marketshare. Clients don’t care that IE is the de facto standard while you want to comply with the W3C standards. They just want something that works.

I think our best bet is to keep lobbying Microsoft and keep stressing to them that some clean up action is required to allow the web standards and the web itself progress.