Saw these on the recent live stream:

Caped Corvo- "Civilians won't report a hero." Save 10 civilians from the City Guard.

Gimp Suit- "Only a masochist would wear something that makes them easier to detect." Observe a six scene without being detected.

TF2 Spy- "I spy with my little eye." Spy's mask from Team Fortress 2. Steam unlock.

Thieves Guild- "M. Frey is sewn into the collar." Possibly unlocked by having Skyrim save data?

I'm sure there are more.

That's actually really cool. It's like having an extra perk. Definitely wanna see more about these outfits.