Let's say you own a printer at home. Let's say you want to upgrade to Windows Vista or purchase a new PC with Vista. In addition to running into potential application compatibility issues, you may find your printer does not function with the new oprating system. Find out from the printer manufacturer's web site before purchasing! It is possible that the 4 year old printer is being phased out and the manufacturer (i.e. Brother, Xerox, HP) will not provide the needed software (i.e. drivers) for Vista compatibility.
As an aside, keep in mind that this is not the fault of Windows Vista. Drivers are the responsibility of the manufacturer. It's a cruel world, and it may be time to upgrade that printer anyway. However, it is possible that a default or generic PS, postscript, printer driver will in fact work. So you can always hook it up toa new Vista system and see what happens.