I've seen too many people here with problems like this, who claim to be doing everything right, and still their new pi won't boot or won't boot consistently and perhaps even randomly corrupts the SD card. I don't believe they are all fools or liars. There are apparently 40,000 pis a week being made, and we get a question like this at least once every week or so, I think.
A while ago I ran across this thread, where someone claims:
Recently I purchased a Raspberry board labeled Made in China and I have already one board labeled Made in UK. [...] My old SD card image used for Raspberry Pi is not working with the new one.
If a card works in one rev. A pi but not in another, the second pi is broken. Of course, it could be faulty anyway, even if it is made in the UK. The Chinese ones (which are no longer being produced) have a black headphone jack, the UK ones are powder blue.
There is so much material already online about creating and using the SD card -- and it is very simple -- I do not see a point in repeating it here. If it doesn't work, it doesn't work.
You could obviously try another SD card to rule that out, etc. The way I would deal with this, if you decide it is the pi, and you can afford the $50, is to order another one, possibly from a distributor that only does the British ones (Farnell/Element 14). If you can't get either of them to work, either you are really unlucky or doing something wrong. But if one works and the other doesn't, you have a pretty good claim to send one of them back.