I assume you're talking about the seatbelt flapping in the wind when you have the top down or the windows open?
Yes... definitely.
Why they couldn't have built a little bump out in the trim piece at the door (bottom of the interior of the "B pillar"... if there truly was one in a NBC) to protect the seatbelt from the wind is beyond me. I've thought about just adding something myself.
Another solution might be either a strip of velcro, or a couple of plastic clips (like the ones on the back of cell phone holsters).
But, yeah, out of the box, the only solution is to fasten the seatbelt even if there's no one in the seat. I had the service guy tell me this... then in the same breath said "but don't do that because it'll screw up the airbag sensor". Well, then, not much of a "solution", now, is it?!? I've since learned that this isn't true anyway... at least for 2003 NB's and above. The seatbelt does not affect the passenger SRS system... it's all done with weight sensors in the seat now.