I have a RPi Model B, and the green LED is just a small dot. I have already checked all the steps at http://elinux.org/R-Pi_Troubleshooting, but none work.
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I suspect your SD card's image is not correct. If you have a Pi that boots correctly, try its card. If not, try rebuilding the image.
I don't know what version of Linux you're trying to use, so I don't know the specifics. But you might be able to take your SD card and mount it in an OS that can read it and look at the /var/log/dmesg.log or maybe /var/log/boot.msg on the card. You should be able to find the boot messages if the boot process even starts.
Since your LED is 'just a small dot' it could be that you have a faulty power source. Try a different power cable (a Blackberry charger is what I use) and test your current power source on a different device.
If that doesn't work, then you probably have something wrong with either your Pi or your SD card (as mention in another answer).
Are you sure you've gone through everything because this sounds awfully like your problem...
I would use a combination of the answers from Paxton Sanders and user12321. I would try a different power source (I use an iPhone charger with a micro usb cable) and a different image on another SD card. You can get a number of different images for your SD card here: http://www.raspberrypi.org/downloads/
Using a process of elimination you can identify the fault, and then rectify it.