One more thing to add: on a bike, your body language conveys a lot. As one or two people posted earlier, Forester points out that lane position and looking back convey as much (or more) information than just signalling (another reason Forester doesn't like helmet mirrors, btw).
I heard that at the new ADT Velodrome, roadies new to the track were using the flicking elbow to indicate they were going to swing off. Roger Young (the very experienced cycling coach) says that instead of doing this, you just need to look over your shoulder; this conveys the message that you're going to be going up.
And back to lane positioning: this is one thing bikes can do that cars can't - you can occupy different positions in a lane. So if you're at the left edge of the right lane, this is a good indication that you're going to go straight at the next intersection, or if paired with long looks over the shoulder, that you're gong to be moving left.
The worst thing is to not look back, stick out the arm, and start moving, as if the signal is going to protect you. Or, what I hate about drivers - the signal as a formality: initiating the turn, then flipping on the turn signal! I just hate that! The idea is to let us know what you are going to do, not to tell us what you are doing now! I just don't understand you humanoids sometimes...

Luis