Well, first of all, I won't insult your intelligence by saying "You did turn off Adblock, didn't you?", because it's probably only me that forgets about that kind of thing.
Next possibility: Is your blog actually on a public-facing server or is it on a test machine? If it's on a test machine like http://localhost/ then Google might not send the ads. Note that the Google ad robot comes to your page to have a look and decide what ads to put there, and you'll end up with public service ads if it can't see the page.
Third time lucky: I think they say somewhere that it takes ten minutes but sometimes it can take longer. I'd give it a day or two - you probably weren't going to break the bank in that time anyway.
If none of the above does the trick, I'd try making an ultra-simple HTML page like
<google ad here>
and see if that works on your site. If that doesn't work, then I'd contact Google.