First impression: Confused. Do you mean will using metal can be harmful to plants? The answer is no. Do you mean are cans useful to start seeds or cuttings? The answer is yes. Do you mean if you use cans will your yard look like the Beverly Hillbillies live there? That is your choice to make!

Sorry, guess I didn't make the question clear enough. What I meant was, if I plant in the cans, will the metal that the cans are made of harm the plant that is planted inside? I have been saving cans to use to start seedlings in, and have been scared to use them, because I didn't know if the metal would somehow harm the seedlings when they started to grow, especially if they started to rust. It seemed like such a good idea though, some of those cans are just the right size for certain things that I intended to put in them. Thanks for your help.
(P.S. There's no hope for my yard; I am married to a sheetrock hanger, and my yard has looked liked the Beverly Hillbillies for 30 years; I'm laughing hysterically over here! Too funny!)

The cans should be fine as long as you punch drainage holes in the bottom. Also, consider how you will get the plants out when they outgrow the can. Paper cups, yogurt containers, etc, can be torn or cut down the side to minimize damage to the seedlings. Otherwise, have at it.