You are required to sign your application with a certificate issued by a trusted CA in order to authenticate that it is indeed your own application. It is similar to the signature you put on a letter, as long as the signature is recognized as being yours the recepient of the letter with trusts its content. But just like in a letter, the signature does not tell that the content is true (or that the app does not eat all the RAM it finds, to follow your example) but that it comes from you.
On the other side, the certification means that your application is signed by a 3rd part which guarantees with his signature not only that you have been verified to be the source of the content but also that the content is true (or for apps that they obay minimum quality criteria).
Now, one things I would expect to be true for a very long time: there will be no unsigned apps in the Ovi Store