To expand upon why it's such a bad idea...
The thing about TPM is that once you start storing your keys in the TPM you're now tied to that hardware permanently. If the TPM dies (or any other non-trivial part on your motherboard for that matter) you're screwed. TPM-based systems like BitLocker get around this by encrypting copies of your key and encouraging you to make USB keys, store backups etc.
Of course the more backups you keep the more backups there are to get stolen. Since the keys in your wallet.dat are essentially full access to your entire account, their compromise means the complete and irreversible theft of your bitcoins.
Now I'd get behind a config option for the bitcoin client to store wallet.dat elsewhere (perhaps inside a TrueCrypt volume which I've stored on DropBox?) but TPM is just a really terrible idea.