Eidos Montreal's new Thief has had a somewhat strained relationship with fans of the series. Under normal circumstances, you'd think this announcement could alleviate some of the worry: it's a bank heist mission, advertised as a "true homage" to Thief II's very own bank-based escapades. That sounds like a significant and meaningful connection to the series' past. So of course it's pre-order DLC.
It's a bit premature, given that the game - which is due out on the 28th Feb, 2014 - doesn't seem to be available for pre-ordering yet. Even if it was, I'd offer the same advice that we always do: don't. Pre-orders are a needless risk, whatever the game. Thief could well turn out to be brilliant. If that turns out to be the case, you'll almost certainly be able to buy this mission as a separate DLC pack post-launch.
Eidos Montreal have pulled this trick before, with the "Tong's Rescue" mission for Deus Ex: Human Revolution. If it's any consolation: that sidequest wasn't very good. This one does sound slightly more promising, with security measures unique to the mission, and what appears to be a heavy focus on the thieving aspect of Thief.