I can make it open spread out on two monitors by using blender.exe -p 2 86 2946 1000 (those numbers work for me on a 1680x1050 next to a 1280x1024).
However Blender is always resizing and when I click on any menu I get garbled artifacts instead. So basically it is unuseable for me.
People on some forums have gotten better results, I guess it depends on your video card.
*BUT*... I've heard Blender 2.50 WILL support multiple windows !!!
