The big irony is that, by doing this, Microsoft have set up the ideal conditions to marginalise their own browser. Clueless developers won’t know about this behaviour so every new site they build will automatically be rendered as IE7. Clued-up developers will use this as an excuse to freeze support for IE and turn their attentions to better browsers.
Food for thought. Andy Budd’s account of the problem with the IE team’s decision for version targeting is, so far, the most thoughtful one. Worth reading.