is this planned to be implemented? Would be great to be able to sign transactions securely on an air-gapped machine.
I understand that IceBox has offline signing, but they don't support contracts/multisig and custom tokens.
You could try using MyEtherWallet (https://www.myetherwallet.com/) where the source code is available for downloading to an offline computer from https://github.com/kvhnuke/etherwallet .
A note from their "Advance / Offline Transaction" page - "This tab is brand-spanking-new. Please report any bugs / issues in the reddit thread & please test with SMALL AMOUNTS FOR NOW! Thank you!"
Nice work https://ethereum.stackexchange.com/users/300/tayvano .