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Lion may not come preinstalled on day 1, but the install disc will be packaged on all existing models at date of Lion launch.
I even think Apple will offer up to a 1 month grace period before Lion's release for new Mac purchasers. They did this for Snow leopard when I purchased my mid 2009 MacBook Pro in July. I was offered a free copy of SL with the exception of $10 shipping.
So don't worry, as long you purchase close to Lion's release.