Shortly after Jaguar came out, I bought a 700mhz eMac from the online Apple Store. The eMac came with 10.1 installed on it and an upgrade CD for 10.2 in the box. You had to completely install 10.1 first in order to install 10.2; there was no clean install option.
Machines that debut with Tiger, like today's newly announced PowerMacs, will probably include the full installer for Tiger.