...Display Port has the ability to output both the DVI digital signal (DVI-D) and the VGA analog signal.
Are you sure about that? Somebody previously corrected my misunderstanding of how a DP-VGA adapter worked, which contradicts that.
DisplayPort has the capability to handle "active" adapters; these adapters draw power from the DisplayPort port itself (or, occasionally, another power source like a USB port; DP can supply about 1.5W of power), and take the digital signal from the DisplayPort output, then convert it internally to a different type of signal, which is then sent to the monitor.
I was under the impression, and both the Wikipedia page and the
official DisplayPort FAQ seem to explicitly confirm, that a standard DisplayPort port is NOT capable of outputting an analog signal. Rather, DisplayPort/MiniDisplayPort to VGA adapters do the digital-analog conversion within the adapter. This is different from DVI-I, which carries both a pure-digital DVI signal (DVI-D), and a separate set of wires for an analog VGA signal (DVI-A), meaning all you need is an adapter that re-routes the wires to get a VGA signal out of most DVI ports.
DisplayPort ports can, however, rearrange their output signals to mimic a DVI-D or HDMI port, if a passive adapter tells them to do so and directs the signal wires correctly. Active adapters to do both of these things also exist, however (in which case the DisplayPort is outputting DisplayPort video, and the adapter itself is converting it to DVI or HDMI), and if I understand correctly are the only way to get a dual-link signal out of a DisplayPort connection.
I assume that passive adapters are cheaper than active ones, and while it's theoretically possible that some DisplayPort GPUs are capable of handling a passive VGA adapter by outputting an analog signal directly, I don't believe Apple's MDP ports do so.
I'd love to know if I'm wrong about this; I do like to keep on top of the real details of how various connection standards work.
[Addendum: Indeed, I just cracked open my cheap (<$20), off-brand Mini-DisplayPort to VGA adapter, and contained within is an Analogix ANX9832 chip; that chip is expressly designed as a low-power DAC to convert the digital signal output by DisplayPort into an analog VGA signal.]