Has anyone figured out a good way to draw an arc in Keynote?

This can be done in Keynote -- it is a bit cumbersome, but offers a huge amount of flexibility in the final product:

- In the Shapes menu in the toolbar, choose the Draw an Arbitrary Shape option (the last one with the pen).

- Click once to start a line, a second time to produce an endpoint, and then press Escape to stop drawing. You will now have a line.

- In the Graphic Inspector, set the Fill to "none", the Stroke to whatever colour you want the line to be, and use the Endpoints menus to add an arrowhead.

- To make this line an arc, select the line, then in the Format menu, under the Shape flyout menu, choose "Make Editable". The line should now show a red dot at each end, which are Bezier control points and you can now add a third Bezier control point to the line, which is how you will make it arc.

- With the line selected and the control points visible, hold down the Option key. Your cursor should change to a pen nib with a plus sign, indicating that you can add a control point on the line. Click on the middle of the line to add a point.

- You can drag this point with the cursor, and it will create an angle in the line. By default the point is a corner, and so the angle is sharp. Double click on the point to make it a smooth curve point. You will now see two Bezier control handles on either side of the point tangential to the curve. You can move the handles to change how the curve looks (such as how tight or wide the curve is). You can manipulate each handle individually, or hold down the Option key to affect both handles.

This procedure sounds more difficult than it actually is in practice, and the arc you get can be completely customized as you like. That said, if you find you use the same basic shape over and over, you might consider creating a "clippings" presentation that has various re-used elements in it, so that you can easily copy and paste from it to other presentations.