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Ummm, yes.
It seems to me miners will have an incentive to lie about the transaction ledger, and put fake ledger hashes in their blocks. Either so their transactions might be considered 'unspent' by unsuspecting nodes that trust them, or so that other miners that don't have the full block chain create invalid blocks (eliminate the competition!)
And I don't see a proposal that everybody check the ledger and reject blocks that contain invalid ledger hashes.
I also don't see what the ledger hash accomplishes. If you're going to trust some other node's version of unspent-transaction-reality, then you could just ask "send me the ledger state before (or after) the block with THIS block hash".
But if you're going to trust one or more nodes anyway... then it seems to me sending an ever-increasing-in-size ledger is a bad way to get scalable. If size-of-full-blockchain becomes a problem before the mining pools and big exchanges/merchants/transactions processors all have transaction processing clusters with a terabyte of ram and petabyte hard drive array then I think extending the protocol to make it easy to request all transactions involved in a given Merkle branch will probably be the way to go.
But before then I expect the bitcoin network will look very different from the way it looks today, and I expect there will be several different solutions for how to scale up. If (when!) Bitcoin gets that successful, there will be serious money hiring the same smart people who figured out how to scale up PayPal and Visa.