I'm in Savannah, GA - I actually just posted on this (before I saw this post). I'e heard three over the past two months - the first time I heard it, I tweeted it (I since deleted the tweet, but you can google isms boom SAVANNAH and see it still there.)

I heard the first boom Feb. 16th. The second boom I didn't tweet or otherwise note, the third was March 16th, exactly one month after the first. The third boom found me here looking up booms and then googling - others have reported it too, and it made the local news.

[link to www2.wsav.com]

For the record, we are located near Fort Stewart and I'm very familiar with sonic booms (you hear those a lot). This definitely was not that.