Forehead kisses feel very intimate, i'd almost say more intimate then a tongue kiss in that they express a deeper relationship. It would feel strange to me if a girl would kiss me on the forehead if our relationship didn't reach a certain depth.
Depend on the person you kiss. If the person you kiss has some viral infections(small ones like cough, running nose, etc), it may spread. Don't worry about big or dangerous diseases, they don't spread usually(except someone with H1N1[whom you probably won't kiss]). Anyways, whatever you're up to, good luck!!!