A friend of mine just got a Mini, and new Macs come with Tiger. But it still had 10.3 preloaded and he had to upgrade from a DVD included in the box. Previous Macs had a sort of copy protection that would prevent you from using in-box OS media on different hardware than it was shipped with, but I don't know if that'll be the same with the upgrade disc.

So, in short: maybe, maybe not.