Lubuntu is a Ubuntu variant using the LXDE desktop. It’s designed to be a lightweight and easy-to-use desktop environment. Lubuntu is currently not part of the Ubuntu family, and not build with the current Ubuntu infrastructure. This release is considered as a « stable beta », a result that could be a final and stable release if it was included in the Ubuntu family. Please note also that Lubuntu 10.04 is not a LTS version. For the future we aim to include all packages of lubuntu in the Ubuntu repository.
Lubuntu still contains modifications not available in the official repository, you can see them in the Lubuntu PPA  :