FYI: Armory doesn't do this yet. If you have an older wallet, you can migrate it into Armory, but Armory will start generating different addresses than Bitcoin-Qt/bitcoind after that point, so they won't really be the same wallet anymore.
However, the main devs have decided to implement deterministic wallets into Bitcoin-Qt/bitcoind, and I've been part of the development discussion about it. I will be implementing the same wallet format (and maintain support for the old wallet format, too), so it will be possible to share wallets between programs. That's probably a few months away, though.