Sooner is Better (Score 2, Insightful)

There are those who feel that maintaining the old rendering mode would be preferable - allow developers to add the meta tag forcing standards mode. "Don't break the web."

The problem is that unless 100% of new pages include that tag, the amount of broken stuff out there keeps increasing.

Assuming the goal is write-once, standards-based content (please tell me that's the goal), you can break it now, or break it years from now when the amount of content has grown.

Do it now, rip the bandaid off. Force future user agents to use the One True Markup so we don't end up in this situation again.