I remember reading about how these little key chains with real plants inside were a big thing in Japan a few months ago and now they're starting to show up in American shopping malls and probably teenagers' cell phones.

The Korean company Fingerose makes these pet trees and are the brand I've been seeing most frequently in the little stores that carry Sanrio-ish products and stationery. I don't see any stores that are selling these online, but they are coming, not to worry, and they cost about $10 - $15, depending upon the plant. There are cactuses, tiny bamboo trees, even orchids. I've seen some on Ebay but with predictably marked up prices.