I just got my Pi today (the 512 mb model, make in the UK), and I've so far been unable to do anything with it. I plugged it in to my TV using my hdmi cable, hooked up a super simple keyboard, and powered it with my cellphone charger (which supplies 1A). The 4GB SD card was loaded with the Raspbian wheezy image from this page, and followed the directions here to set it up. When it's powered up the LED comes on, but nothing appears on the screen. What am I missing?
The Pi will display a blank screen if it is unable to boot from the SD card - mine behaves exactly the way you described if I try to boot it with no SD card inserted. There may be something wrong with your image or it may not be written to the SD card correctly.
The easiest way to check if this is your problem is to unzip BerryBoot to a FAT32 formatted SD card and boot off that. It's quick, easy, hard to mess up and if it lets you boot you can download/install Raspbian over wifi/ethernet/USB. Either way if it loads up you know your Pi is probably O.K.
You might also like to check that the SHA-1 hash of the image you downloaded matches the one displayed on the website to see if it downloaded correctly. If the hash matches you could also try re-writing the image to the SD card; trying a new / different SD card; or testing the imaged card on a different Pi or within a virtual machine.
The issue may be with the HDMI display. You need to change the settings in the config file. refer to this link - http://www.raspberrypi.org/phpBB3/viewtopic.php?f=26&t=5851
Hope it helps.