According to this post on the US PS boards, Sony's promising to deliver a further 25,000 LittleBigPlanet beta keys in the coming weeks through a variety of websites.
Just remember: don't judge it.
"This beta is being released in order to test element of the online game," said the poster. "As such it should not be seen as a demo, a trial or anything more than helping us to ensure that the server architecture is running. If you don't get a key you are missing out on a version of the game which is extremely limited and not fully representative of the final Blu-ray release of the game.
"If you don't get into the beta, sorry for the disappointment, however the full game is not far away and in the meantime you can (enviously, I know) read many hundreds of reviews from those who are helping with the testing."
You can read our impressions here.