Tomorrow marks the release of the Citadel downloadable content ($15) for Mass Effect 3 on PC, Xbox 360, and PlayStation 3. I'm not so sure about this new "threat," but I like seeing so many familiar faces return. Frankly, there's no such thing as too much Wrex.
BioWare has also set up an FAQ that talks about when you'll be able to access Citadel, the fact that the main ending won't be impacted (outside of the ensuing increased Galactic Readiness score), and that on Xbox 360, the add-on will be split into two separate downloads -- it's that big, apparently. The first file will be $15, while the second is free; just be sure to grab both.