The network stake will never be more than a fraction of the total money supply as coins used for staking are essentially locked capital. A coin with 100% of the money supply being used as a stake would require 100% of the coins to be in hot wallets not being used for anything else (no cold storage, no transactions, no economic activity).
I don't believe that is true for Nxt. Especially with leased forging. ("Forging" is what Nxt calls mining.) Leased forging delegates the forging power of one node to another, leave the source address unable to forge. However, the source address still owns the coins and they can still be spent - spending them reduces the effective forging power of the other node. There's a transaction that sets this up, and then the network remembers the lease and takes it into account when calculating forging powers.
Once leased, the source address no longer needs to be online. Your stake isn't locked capital. You can still spend it and be economically active. You can keep your stake in cold storage and still use it for forging.
This makes the situation worse does it not ? Now NXT owners will lease their coins to a handful of operators to earn fees.
These operators would be perfectly placed to mount an attack. They would have a % of the POS coins and they can mount a double spend attack with pretty much zero risk as identified by DaT.
It would be nice to get a clear statement from the NXT guys as to wether this is possible or not.