There are several shrubs/trees that we can use as possible substitutes for lilacs. I have seen crape myrtles touted as the Southern lilac. They start blooming in June and put on a spectacle for the next month or so. Here's a picture of mine. I don't know which variety this is, but it looks nice growing in my front yard. As much as I love crape myrtles though, they don't seem like a lilac sub to me because they have no smell, which is one of the most notable traits of lilacs.
Buddleias, which are better known as butterfly bushes, can also be something of a substitute for lilacs. They have similar-looking panicles of flowers which smell very sweet. This is the only picture I have of my Royal Red buddleia. These fellows are great here in the South. They are a must have for attracting butterflies. There are many colors to choose from. They are also easy and grow just about anywhere in the United States. In the South, they get big enough to be small trees. Deadhead the blooms when they fade and you get a second flush.
There is one other candidate for a lilac substitute down here, and that would be the vitex or chaste tree. I just got one of these late last year and have no pictures yet. You can look it up on Floridata or Google to see what it looks like. It's another old favorite that is not seen often anymore. Vitex comes in colors blue, purple, and white. Blue is its best color. It has a nice fragrance, gets to be a small tree about the size of a crape myrtle, and will also rebloom if deadheaded. I would import some pictures of it, but I'm worried about copyright infringement. Just trust me, vitex is a very worthy Southern plant. It's said to be hardy to zone 7. One other great thing about the vitex is that it's very drought tolerant. Check it out and try it. By the way, if anyone reading this has a picture of their vitex tree, I'd love to get a copy to post.
PS - Thanks to another blogger for sending me this picture of a vitex tree to include. Check out his web page at www.phillipoliver.net.