I was actually going to mention this in my "oh hi I'm a huge n00b" post. So, here's what I would've said:

And why has no one invented turn signals for bikes? I hate arm signals. Not only do I feel they look dumb, but I don't trust most drivers to know what they mean anyway since they're so scarcely used. Half of them will probably think I'm flipping them off (we won't go into the safety implications of that). A blinking turn signal, everyone understands.

I thought of this on my ride today when waiting to turn left into the driveway of our complex; another car was coming and I stopped, just as I would've in my car, but the driver probably wondered for a split second why I was stopping. But what was I to do, stand there with my arm out until she was past? She probably would've thought I wanted a high-five!

If there's little traffic, no problem. If I'm already in the left-turn lane, no problem. Simple right turn, no problem. But what about changing lanes in traffic? Maybe it's not as difficult on a bike as in a car, I dunno, but sometimes my turn signal is the only thing that gets me space to move over in heavy traffic (and sometimes it just signals other drivers to speed up so they don't have to let me in--boy do they get pissed when I nudge in anyway! But that is SO not possible on a bike).