To the best of my knowledge, those printers contain no hazardous waste, so they could (depending on your local regulations - check with your city government) be put in the trash. That said, though, it's best to avoid putting anything in a landfill if it can be avoided, so you might check around and see if any used computer store in your area could use them for parts. Check also with any recycling services in your area to see if they'll take them to reclaim any of the materials. There are also national recycling programs, such as this one:
http://www.recycleapc.com/apple/index.asp
There is a charge with such programs, but dumping stuff in a landfill also has a cost; it's just hidden in the various taxes and in the unallocated costs we all incur for resource waste and environmental degradation.
Regards.