Leopard has a better routine to reclaim unused space from the sparseimage file.
In Tiger, it would ask you if you wanted to compact ro not (thus just sitting at the prompt after a shutdown or restart request).
In Leopard, it starts to automatically compact the spares images after a restart/logoff/shutdown. You can cancel it if you can't wait. At least this way you can just reboot and walk away and it'll compact it without prompting.
It used to be an issue for me too, but I've got 1.5TB on my Mac now (250+250+250+750). Add my 1.5TB NAS and I've got 3GB in my home.
My only complaint with Filevault is that under some types of crashes, preferences tend to get corrupted or damaged so apps will reset to defaults. Unfortunately, this has happened once with Leopard as well.

I think it's a delay in writing the encrypted data to the image file that causes this. Really sucks when I have to resetup my dock or finder toolbars (whichseem to be the most frequent victims.