Just got off the phone with Capcom UK: they said they're looking into the issue and if they gave us an ETA, they'd be lying.
Tried Xbox Live support, and they were of zero use. I was asked if I had tried deleting and re-downloading both SSFIV and the add-on. I said yes. We also tried deleting the system cache and deleting my profile/recovering gamer tag.
I was then asked to re-download from the download history, not from Marketplace. I told the guy it would take ages and that I just hoped for a refund for the points I spent on AE, seeing as that's the one not working. He said the issue has been escalated (but earlier said it wasn't "widespread"), and that they'd get in touch within 3 weeks.
Brilliant.
Anyhoo, in order to perk discussion up again, I vote for this gem:
Imm0rt4l said:
Super Street Fighter 4 Arcade Edition|OT| Time to let the twins out