Well, I remember that traditionally, if a man and woman began to "dutzen" one another (use the du-form), it meant they had kissed. I think that's all changed now. I am now more familiar with French habits, where students are all "tu" to one another, and in certain creative circles -- music, art, film, etc. -- tu is used almost exclusively. My rule of thumb is when in doubt, use the formal address -- almost no one will be offended.

Salut, James