It's a question we've been asked countless times in the past week, and we have an answer. Previous seasons of The Shield have been released in January (Season 1 and 2), February (season 3) and December (season 4). The release dates were tied to the premieres of the next season; Season 2 and season 3 premiered in January, season 4 premiered in March, and season 5 went back to January. Studios try to do that so they get more promotion for the DVD sets; TV writers (who don't normally talk about DVDs) may mention "pick up season 3 on DVD a week before season 4 airs." It's a little bit of free promotion that helps move copies of the set.
Okay, back to season 5 on DVD. We haven't heard an announcement for the set because the network hasn't locked in the season 6 premiere yet. Once that's known we'll hear the date for the season 5 DVD set. We've heard rumors that the network is looking at a March premiere, so we'll likely see a late February or early March DVD release.