Recently, I have had this annoying problem, which also resulted in me destroying a Mini DisplayPort (MiniDP) to DVI adapter, because I thought it was broken. I wanted to share my experience so nobody throws their adapter to the bin. I may be oversimplifying things, but it will probably solve your problem.
OK, now the symptoms of this problem: You connect your MiniDP to DVI adapter to the monitor, you open up System Preferences and do a "Detect Displays". The screen blinks, surely your monitor is detected. Even your monitor's model is displayed on one of the window's title. But your monitor is still blank. It also happens if you connect a monitor using a VGA to DVI adapter (Macbook <-> MiniDP to DVI <-> DVI to VGA <-> monitor).
First, basics. The 4 pins (orange) around the horizontal pin are for analog signals (which is VGA signals for our case).
When you plugin a true digital monitor those pins are not used. Some monitors (e.g. my darn monitor) have DVI cables but in fact they receive signals using these 4 pins. When you use a DVI to VGA adapter (Computer <-> DVI to VGA <-> Monitor), again, those 4 pins are used.
If your computer only sends digital signal and does not send VGA signal through those pins, your DVI to VGA adapter will not transmit signals, because it did not receive anything to transmit! Hence, you will not be able to display anything on your monitor. (Okay, some other pins are used so that you see your monitor's model, but the essential pins are those 4 pins)
When you use MiniDP to DVI adapter, your Macbook only produces digital signal (meaning 4 pins are not used). When you connect it to a non-digital monitor with a DVI cable or using DVI to VGA adapter, it will not work. Damn. The only way to make it work is to buy a MiniDP to VGA adapter (or a new digital monitor).