The slowest part of the stand-alone Mac OS X install are languages and printer drivers. The total size of the drivers as far as I remember is 3-4GB and the granularity of the selection is very limited. You can select all HP, all Epson printers etc. All vendors are selected by default. There is no easy way to remove the unused drivers. This approach may be supported by the idea that, once installation is done, it will provide better user experience when connecting to new printers. This might be important for mobile users.
With Leopard, Apple played with a different approach - when a new printer is detected but no driver is found, auto-update checks for a driver. This was made available for HP printers only (as far as I remember). The default installation, however, was still installing all drivers.
With the disks that ship along the new hardware, another slowdown is the bundled software. Currently, after the user selects or deselects certain software, a standard package for that item is launched. The package checks for the OS version, installed components etc. Theoretically this could be improved as well - both the hardware and OS version is known - but would require additional QA effort and might not be worth it.
I believe Apple will improve installation process with Snow Leopard. I expect some changes both in language support handling and printer driver install process. But, let's face it, considering the fact that Apple is not pushing enterprise sales, this is not going to be a priority.
A problem which can not be addressed is the slow reading speed of the SuperDrives, especially on portables - a trade-off for slot-loading and DL write support.
I am pretty sure everybody on this board will trade installation speed slowdown for OS speed or boot-up speed improvement.