A USB 2.0 Device in a USB 3.0 port will will run fine, but only at USB 2.0 speeds, making it pointless, really.
Unless you have no other USB ports free, of course.
There is no reason not to, but on the flipside, apart from lack of ports, there is no real reason to do it, either.
The early 3.0 Memory Sticks/Externals have been trickling out over the past few months, but I still think it's a long time before it becomes the new standard.