That's an interesting point. I travel on KLM a lot and they announce, when they're discussing what electronic equipment may or may not be used 'Equipment with an antenna may never be used'. So I guess that, since technically Bluetooth does use an antenna, it would not be allowed.
But the situation is certainly in flux because, of course, several airlines now offer on-board WiFi for in-flight Internet access. So it's clearly currently a carrier by carrier situation - some allow, some don't.
As the last poster pointed out, most airlines prohibit the use of any electronic equipment during taxi, take off and landing.