^I'm in a similar situation with my infiltrator playthrough. Picked it up again last week after not touching it in months, managed to get from the Cerberus coup to the Thessia mission (doing Leviathan along the way) and was about to start it--with the intention of doing Omega after Horizon--when this new DLC was announced.
Now I think I'll wait since this DLC probably wouldn't fit very well in the latter third. Indeed, from what I gather, unlike other DLCs that unlocked right after the prologue missions, with this one you'll have to wait until *after* the Cerberus coup. Makes sense, based on the description, though I'm sure fans of Thane as a LI might feel a little short changed. Still, past experience has taught me to temper my expectations, especially when Bioware rave about how good they think it is.
Another little tidbit I found interesting; apparantly the Lancer AR mentioned in the MP DLC is to be the same Lancer from ME1, cooldown mechanic and all. One assumes that the design will just be a re-skinned Avenger.