Does anyone have any idea why that would be happening?
EDIT: The commenter just posted the Mt Gox error: "This address is not on the right network"
Armory and Mt Gox, and all bitcoinj-derived wallets, don't support P2SH addresses yet. Electrum does support P2SH, and so does Bitcoin-QT.
P2SH was added about two years ago - about time wallets upgrade...