Definitely a mempool.bin issue. Armory attempted to broadcast it, but Bitcoin-Qt was probably disconnected from the network when it tried, and now it's stuck (for reasons that are difficult to describe).
Close Armory
Close Bitcoin-Qt
Remove ~/Library/Application Support/Armory/mempool.bin.
Restart Bitcoin-Qt
Restart Armory
Send transaction again
If this was an offline transaction, you don't even need to recreate&sign. Just select the file in the offline-broadcast window by "Show All Files" at the bottom-right of the file-select screen. Select the *.SENT.tx (normally you select a *.unsigned.tx or *.signed.tx).
If you are using an older version of Armory on the offline computer, there may be other issues, but those will be resolved with the new version of Armory posted on the
RAM-Reduction testing version thread.