What's a DVI-to-VGA connection have to do with sleep?

Question

I've been experimenting with hooking my HDTV to my Mac Pro's 5870 video card. Currently I have my 27" ACD connected to one of the mini displayports (mDP) and a VGA, connected through an Apple VGA-to-mDP (active) adapter to the other mDP. When I want to use the TV I can connect the HDTV to the 5870's DVI connection with a (passive) HDMI-to-DVI adapter. It works. Indeed it works too well! Whether the TV is taking input from the computer or not the computer thinks the TV is a third monitor which can get very confusing when the mouse is accidentally moved off my ACD on to the TV (6 feet away) and I can't find the cursor. Another minor nit is that using the DVI means no audio can be passed on the HDMI so the computer speakers are used unless of course I hook up a separate audio connection.
I could get around the 3-monitor problem using SwitchResX but I prefer not to. I'm willing to simply manually connect and disconnect the HDMI cable on those few times I want stream stuff to the TV. But as an alternative I tried switching the TV and VGA connections, i.e., connect the TV to the mDP through a (passive) mDP-to-HDMI adapter and the VGA through a (passive) VGA-to-DVI adapter.
This second scheme also works. However because all the adapters are now passive the VGA will disconnect when the HDMI is plugged in. Ok. I can live with that and I understand the behavior due to the 3-monitor limitations of the 5xxx cards (i.e., active adapters required).
All the above was just background and context leading to the question I entitled this post with. Forget the HDMI connection. I have the ACD connected to its mDP and the VGA through the passive VGA-to-DVI adapter (i.e., the second scheme I described above). With this configuration my machine will not sleep on its own (i.e., at the time the energy saver specifies). I can "manually" put it to sleep ("Sleep" from the Apple menu). It just won't do it on its own (and none of the checkboxes in energy saver are checked -- wake for network for example -- and no active processes are running that would explain this behavior either).
Experimentation has shown that it is somehow related to using the VGA through the DVI. If I put my original configuration back (VGA-to-mDP) the machine will sleep on its own at the designated energy saver preference time.
What I don't understand is what's a DVI connection have to do with the energy saver sleep setting? The only remote possibility I can think of is perhaps the screen saver is running on both the VGA and ACD and an active VGA is sufficient to make the machine think there's activity when connected through the DVI (but not when connected through the mDP).
Of course none of this is really a serious problem. I'm only asking for academic interest.

Answer 1

I know I can do that but that's not the subject of this thread! I'm only asking why would a VGA connection to DVI keeps the computer from sleeping on its own.
And, oh, by the way, another audio alternative is something like this,
http://www.gefen.com/gefentv/gtvproduct.jsp?prod_id=5279
although this requires stringing a separate audio cable.