Thanks for the input.

1) It is possible to print money in a 51% attack if other users don't have the full history. 51% Attacker outruns the whole chain by more than the month that everyone does store, so that NO-ONE has the history. Then you can do what you like. not very likely I agree.. (Outrunning a month with 51% takes Years)

2) You can still verify the longest chain via POW even with this maximal pruning. It is not a blind trust-the-peer situation.  

3) A peer cannot tamper / alter / change the data he is providing you - because.. Hashing!. Either it is the chain data or it isn't. At that stage I would simply go with the chain with the most POW.

4) The man in the middle attack - where the attacker cuts me off from the valid network, so I only see their chain, is a concern even without the pruning. 

5) As long as you keep up with the network, log in once a month, you have the _same_ security as non-pruned bitcoin - as you still validate the whole chain.

I think what this does is change the requirements for the network from - everyone needs a big hard drive - to - everyone needs a small hard drive and to log in once a month. Fair enough.

I agree that if you miss your 1 month window.. you'll need to place trust in the longest POW chain, but that seems like a given anyway.

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EDIT : Transactions from years back would still be available as you could provide the merkle proofs that linked to the block headers with the original data. You'd have to store them yourself though.