The more I think about this theme, apple could wait a couple of weeks after the ML release:
When ML shows to work fine, customers want to buy new imacs, either they waited for the new mac, either they migrate from Windows (or Linux) or their current imac has not enough performance to have the real fun with ML or is to old.
And when it is not coming ,ok - I migrated from Linux to mac, I can proceed to anything else.
See I don't agree with you because last year they introduced the mac minis and airs with lion and lion was a bigger update than ML. So they way you discribed introducing the machines after to see if it works right doesn't hold any weight in apples eyes