Dave Walker at the New Orleans Times-Picayune is the man I trust on all things "Treme," and he has a significant piece of news today: HBO has delayed the "Treme" season 3 premiere date until sometime in the fall.
The first two "Treme" seasons aired in the spring, and the show was absent from HBO's list of spring premiere dates from press tour. Given that "True Blood" owns the summer for HBO, "Treme" was either going to have to air on a different night (which has not been a boon to other HBO shows like the canceled "Bored to Death") or be delayed.
Presumably, "Boardwalk Empire" season 3 will also be on in the fall, and HBO still has to schedule Aaron Sorkin's drama about cable TV news, so "Treme" could wind up airing on a different night, after all. We'll see. David Simon has told me and Dave that ideally the show would run four seasons, but that'll be up to HBO to decide. The ratings for the first two seasons weren't particularly strong, and at the moment it's a show that exists because HBO likes it and wants to be in business with Simon. Hopefully, whenever it airs in the fall, they'll still feel that way about it.
UPDATE: HBO has asked me to clarify that the premiere hasn't technically been delayed, since they never announced a premiere date of any kind for the season. Everyone just assumed it would air in the spring, since HBO usually - but not always - airs its various shows at the same time each year.