How to warm plates before serving?
We do this regularly with regular dishes and fine china. Neither say "oven safe". My mother has also done it for years. The process is fairly simple:


Put our oven on it's coolest setting (about 175°F or 80°C, I think)
Wait for it to reach temperature 
Turn it off
Put dishes in and close the oven. We just stack the dishes. As Joe points out, they'll warm faster if you don't stack them. However, I usually do this when I've got 6-8 dishes, so stack is easier. 


Be careful when taking the dishes out. They'll be hot. We've never had a problem with dishes using this method. However, if you don't think your dishes will stand getting hot, use your own judgement as there's a wide variety of quality and materials used in place settings.