Under IIS7 the Etag change number (the part of the Etag following : ) is always set to 0.
Hence the Etag from the server no longer varies from server to server for the same file and therefore the Yahoo best practice no longer really applies.
Since you can't actually suppress the ETag header on IIS7 it would probably be best that you don't fiddle with it at all. I've found by far the most useful configuration rule is "If the default doesn't break something, leave it alone".
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