RE: $100 per-transaction, $500 balance limit:
That makes perfect sense; they probably figured out about how much it will cost to hack a MintChip to get it's private key (dissolve case in acid, put it under an electron microscope, attach electrodes at exactly the right spots, etc...). Do a little calculation involving the cost of hacking one chip, the number of times you can double-spend before you're likely to get caught and the maximum amount per transaction and I bet they figure it doesn't pay.
Especially if online transactions "phone home" to detect double-spends. If you have to physically walk to 500 different not-online merchants to get away with $50,000 worth of double-spends that's just like counterfeiting $100 bills, and that's an attack Mints have been pretty successfully dealing with for hundreds of years.
RE: anonymity: the anonymity model is similar to Bitcoin. Each physical MintChip is like a Bitcoin keypair, if you can easily buy/load a bunch of them anonymously then it will be hard for Them to track your purchases.
If MintChip fails I bet it is not due to hacking or lack of anonymity, but just due to the inconvenience of needing Yet Another Physical Doohickey. Paper money fits nicely into the wallet I already have, I don't want Yet Another Dongle on my keychain, and I bet before the end of the year either Apple will finally approve a Bitcoin app or there will be a nifty HTML5-based web wallet I can use on my iPhone...