They could certainly send me a fake chain, but it would took them more than millions to convince me if I simply check the difficulty. The whole point of Bitcoin as stated in Satoshi's paper is voting by IP doesn't work, malicious nodes shouldn't win just by numbers.
To send a fake POS chain, they need to control more than half of the stake from before the lock-in.
The point of the POW stage is to make sure that this is well distributed.
Obtaining the POS coins would be made harder, since a lot of the owners of that stake would no longer be on the network and/or would have lost their keys.
The devil is in the details though and PPCoin is the only attempt to do it and that isn't a pure POS coin.