It seems to be possible, at least a lot of customers of my provider congstar(germany) report that it did work for them (http://www.facebook.com/congstar), even without using sand paper to make the sim card thinner. The "golden card" you see are simply the contacts of the much smaller chip ( see http://de.m.wikipedia.org/w/index.php?title=Datei:SIM-Card_X-ray_contrast.jpg&filetimestamp=20101219232326), so it's ok to lose some of that contact area, which is what you have to do in order to make your sim card match the nano dimensions. There are already several templates and how-tos available. Just google it and try it -- at your own risk.
PS: Many providers anounced that they will make nano sims available very soon, so you might want to contact your provider and wait a few weeks :-)