This is great, I always have dozens of tabs open in Safari, and I find the AppleScripts for saving and reloading the tabs to be, well, unreliable.
Now, Forget Me Not is a Safari plugin that remembers what tabs you had open and re-opens them when you launch Safari again. It requires you to instal SIMBL (Smart Input Manager Bundle Loader) though. And there’re some security concerns about Input Managers.
But it seems to work for me. There’s also a Firefox/Mozilla plugin which does a similar thing.