The forum locale wasn't directed at you, Koala. The other user I replied to was having issues on his MacBook Pro apparently, and because they hijacked your thread, I responded directly to them about their misplacement of their post.
Which video card did you get in your MP09? (the GT120 or the ATI 4850?)
Are you attempting to use both ports at the same time, or just one?
If you are using ONLY one display, the mirroring option should not be available to you, as OS X will not have detected multiple displays during startup, so there is no reason to offer mirroring. =) That comment was meant more for the MacBook Pro user that hijacked your thread, since notebook computers have a built in display by default. (1 + 1 = 2 displays. =D )
Have you tried using JUST the DVI port, DVI cable, and setting the monitor to DVI input ONLY (not the auto setting) yet?
If you have the ATI 4850 like I do, I cannot see why your setup would not work when mine took literally 20 seconds to plug in and get running.
Also, the reason you are not seeing the native resolution of the Dell is because 1600 x 1050 is not a standard resoltion. When connected via analog (VGA) OS X only will display 'standard' resolutions as defined in the monitor definition files. (1600x1050 is not included in the default files) When you use DVI, that information is communicated DIGITALLY to the video card, which is why it shows up in the list. (you possibly already know this though)
As mentioned above, remove all connectors from the graphics card, plug just the bundled DVI cable in, switch the Dell monitor to DVI input in ONLY (not auto) and start the MP09 up.
Report back what happens when you do that.
E.