We just heard back from Twitter via email, and they said : “Spamcloud hit. We’re working on restoring accounts.”
We just need to be patient. The suspended accounts will be restored.
Update: We have no idea exactly what a “spam cloud” means. It is probably Twitter staff lingo for a massive spam attack.
Update: We have seen during previous spam attacks that Twitter tends to shoot first and ask questions later and indiscriminately lay down carpet bombing when their system comes under a spam attack. A lot of innocent-bystander accounts get massacred in the process, which they then have to restore afterwards. This is most likely what has happened again.