It is amazing,how one can easly bash somebody else... First it is not lock in to one browser,but rather a info what render is to be used!If the site is accessed by other like FF ,they are free to ignore and render as they wish.Even if this won't be implemented by others it is still usefull as long as bad webmasters are and compability with old unmaintained sites is wanted.And webmasters including texts like "You are using IE,switch to FF like cool people here!" are bad as well(specialy when they put it in javascript,which puts that on top and moves entire content down).

Klimax Danielix

Of course it is lock-in... Or you do not know what lock-in means. If IE would obey public standards what is the reason of using special rendering hints?

Think twice before writing...

Think you missed the point. IE8 breaks compatibility with IE7 and 6 by providing more standards support. The point of X-UA-Compatible is to allow the majority of websites and applications which sniff IE out and render in a special way to run as they currently do without breaking. Then if you happen to want that full-on standards experience, you can just drop a header into your response.

This will work great providing that IE8 doesn't once again have a strange concept of what pass for standards. At least if it does, we can just carry on as normal and never send that header.

Another version of IE to support means more developer headaches. But this new header is a bit of magic that helps ease the pain. I wrote about it here:

Web Developer Tip: X-UA-Compatibility Header Simplifies Life with IE8
http://faseidl.com/public/item/228970