This basically states what I said about IE8 yesterday. The author of Acid2 made a post about this topic. He is very unhappy with the idea, and states several possible ways of handling the situation and the probable outcome of each.
Therefore I recommend not including the meta tag, or, if you are forced to include it, making sure it says “IE=7”, even once IE8 ships. This seems to me to be the best way to show your support for an open, interoperable Web on the long term.
Even if IE8’s rendering engine can properly handle all the stuff that Acid2 tests for, it might not end up working simply because IE8 will default to IE7-mode unless the special meta-tag is in the page. Based on Hixie’s comments, it’s pretty much guaranteed that the Acid2 page won’t include any special tags just for IE8. MS could put some tricks into IE8 to force it to use the new rendering engine when it encounters the Acid2 test, including hardcoding the URL and looking at a “fingerprint” of the page to see if it matches Acid2. However, these could be very easily worked around, by something as simple as hosting the page elsewhere or obfuscating the page in order to change its fingerprint. It would be pretty funny to watch MS trying to explain why it passes Acid2 on this site, but not on that site…
As stated before by myself, Hixie, Opera, Mozilla (continued), Webkit, and even WaSP members, this is not the way to fix the problem. Even Eric Meyer, who supports the IE versioning idea, spent an hour trying to convince a member of the IE team that the default should be “latest” rather than “IE7” (which is the part I have a problem with). I plan to publish standards-compliant code (or at least make my best effort) and not include any extra special notes so that a single browser knows that I really, really, really mean what I wrote. If the new “standards-compliant” IE8 can’t handle that, then I guess it isn’t really standards-compliant.