I had this problem back when I was running Tiger. I was using the CS3 Beta, and noticed that at the beginning of the day, I had about 20GB free or so, then around 4pm, my mac told me that my startup disk was getting full. I had a mere 1.2GB free. And I hadn't really used my laptop for much that day at all.
Restarting fixed the issue, but then it happened again and again until I deleted the CS3 beta.
Then, about 2 weeks ago, when I was running the Leopard Beta, it happened again, this time when I was CarbonCopyCloning one drive to another. This was taking place just before I was upgrading to the full release anyway... so it didn't matter too much.
I'm just sort of glad that I am not the only person to experience this.