Apple usually stops "functionality" updates for their current OS once the newer version comes out, however they release security updates for a while. 10.4 (and I think 10.3 too) have had plenty of security updates during 10.5's life cycle.
I assume apple will maintain 10.5 for a while, since it is the last OSX to support PPC. Although I agree with your point, it seems that for apple to be taken a bit more seriously in the corporate space, they need to guarantee product support life cycles in writing... (although they may already do that, I have no clue. But I know of large corps that need to sign certain contractual guarantees from software vendors specially to secure, in writing, the lifecycle of the products they are buying).
That being said, apple tends to have a pretty good record regarding the support longevity for their products.
"Was it a dream where you see yourself standing in sort of sun-god robes on a
pyramid with thousand naked women screaming and throwing little pickles at you?"