I ask the question because I'm wondering if I want, or need, to update. Tiger does everything I want, and I see nothing new in Leopard I want.
I often prefer software I'm familiar with, as long as it works well and has no adverse security implications due to age.
I remember reading *somewhere*, can't find it now, that security updates will stop when the second new OS is released; that would be 10.6.
Also, looking through the Apple downloads page, I see no recent security updates for 10.3, which suggests that 10.3 updates stopped with 10.5, perhaps.