Apple announced that NEW machines in 2003 will only be able to boot into Mac OS X:
So far these
machines can boot only into Mac OS X. If your machine isn't listed as Mac OS X only bootable, either your machine came with a Mac OS 9 System bootable CD, or it came on a Restore CD. Check first in the Apple menu -> System Preferences -> Classic that a System Folder is present to load Classic, and then check in System Preferences under Startup Disk if that system folder is present. If there is no System Folder to load Classic and your machine is not listed as Mac OS X only bootable, you need to use the Restore CD or Mac OS 9 CD to reinstall Mac OS 9.
Classic means running Mac OS 9 applications while Mac OS X is running in the background. If you startup your machine and see the Mac OS X logo, then if you run a Mac OS 9 application, you are running Classic.
Booting into 9 means that only Mac OS 9 is running in the background when Applications are launching. If when you startup the computer and see the Mac OS 9 logo, then you are booting into 9.
New machines released in 2003 no longer have the capability to take full advantage of Mac OS 9 only hardware drivers. This means that applications that require those drivers will not be able to run on those Macs. The good news is that most applications today are not hardware driver specific, and will be able to run in the Classic environment on machines that will have only the Classic environment in 2003. Even the old Claris MacDraw and Aldus/Adobe Superpaint run fine in Classic. It means you should encourage your hardware driver developers to develop Mac OS X native drivers if they haven't already.
Older machines released prior to 2003 are unaffected.
If you are concerned that your hardware isn't Mac OS X native there are
these links to help alleviate your concerns:
Mac OS X native software search engines
I've listed places you can get
used and refurbished Macs that are able to boot into Mac OS 9, and so has Macprices. I suggest searching there if you are in need of additional machines able to boot into Mac OS 9.