Up here in the Raleigh area (zone 7) I have seen them make it most of the way through the winter. They seem to be able to handle down to like 28 degrees. Once we get down into the low twenties they give it up. I'm talking about the normal leaved version, the larger fuzzy leaf one doesn't seem to like it under 40 degrees (Tibouchina grandifolia).

I had the regular version come through in a fairly non protected spot for about 3 or 4 years. That said, it emerged so late that it almost wasn't worth bothering with since it would just set buds and then frost would cut it down. They can take a lot of abuse in winter; if you have a garage or basement you can put them there, keep them on the dry side and cut them back in spring and they'll be happier than if they were in the ground. The more light & warmth you can provide, the more water they'll need and the faster you'll get lots of green and buds.

Thanks everyone. I think I'll overwinter the plant in the garage.