IE8 was going to behave as IE7 unless explicitly told otherwise (using version targeting). Microsoft have now changed their mind. This is a turn of events that has surprised a few people. Myself included.
What do you know, Microsoft do listen. Thanks Microsoft.
What caused the reversal? I think that a lot of people put out a lot of good reasons as to why this move would be bad for Microsoft, as well as bad for standards aware developers, and Microsoft listened. I wouldn’t be surprised if the Microsoft people also looked into the practicalities of creating all future browsers with full render and processing paths from all the old IE versions inside (so that IE10 can also render perfectly as IE9, IE8, and IE7) and decided it was simply unworkable.