I learned the hard way, and I'm sure frustrated Canary in doing so, that Armory does not digitally sign in a way that is compatible with the bitcoin-qt method, so make sure you don't sign it with Armory. (My understanding is that compatibility will come to Armory later)
I also learned this the hard way and was in a rush to get bitcoin-qt setup and signed a new message. Hopefully neither of our cubes are delayed because of it . .
The latest version of Armory (0.90-beta) will let you sign a message in Bitcoin-qt compatible format. I installed the Ubuntu-64 bit version a couple weeks ago and it has a signing option called "Bare Signature (Bitcoin-Qt Compatible)". It also supports base64 and clearsign formats.