Hi,
Today, I was testing my RPi, just to check that "everything is alright". I bought a cheap 1A/5V micro usb power supply from china (found on a well known auction website) and writed the official aspbian image. I plugged my new VGA/HDMI flat screen, and booted the RPi. Then.... nothing.
Actualy, to make it works, I had to modify the config.txt file, as said by Aptoitos on http://www.designspark.com/knowledge/what-you-need-get-your-raspberry-pi-and-running :
hdmi_drive=2 config_hdmi_boost=4 hdmi_group=1 hdmi_mode=16 hdmi_force_hotplug=1 disable_overscan=0
After adding/uncommenting such things in the config.txt file, it worked fine (I still have to push the auto button of my screen to tell him something is coming from HDMI).
I can't tell exactly what was the problem (signal too low to be recognized, bad display mode, or something else) but then it works fine, and now I'll have to wait for a USB keyboard 
For more informations about which setting change what : http://www.designspark.com/knowledge/what-you-need-get-your-raspberry-pi-and-running