Mobile phones with NFC are getting more popular every day. Samsung and Sony are selling (Smart) Tags and NFC stickers. The tags are quite a lot more expensive than the stickers. You can print this NFC keyfob and put the sticker inside.

This video gives some background into NFC and NFC smart tags:
http://www.youtube.com/watch?v=yLojs9PJi4I

I am using these stickers (10 for 19.90E):
http://www.verkkokauppa.com/fi/product/3750/dkcrt/MyFoneKit-NFC-tags-tarrat-10-kpl-valkoinen

Samsung sells the square stickers, twice as expensive (5 for 19.90E):
http://www.verkkokauppa.com/fi/product/13035/djtfq/Samsung-TecTile-NFC-tarra-5kpl