Can I use the Trezor hardware wallet for cold storage of ETH?
Update 2017: it is now. Ignore the below.
Firmware Update (1.4.0) for Ethereum Developers
This firmware update also brings Ethereum support to TREZOR. However, this is only the first piece of the puzzle. In order for the users to use TREZOR as a secure storage for their Ether, ethereum wallets have to first implement the necessary components. We are working with Mist on the next step of the process, to bring a practical use of this update to our users.
So...not yet still.
EDIT: At the time of posting this in early 2016 the answer was valid. Since it has been added to the Trezor firmware.
Trezor is not directly compatible yet, but it can be used with a solution like what Quorum Wallet does.
EDIT: See this other topic for alternative hardware wallets: Is there any hardware wallet available for Ethereum?