The build infrastructure is not annouced yet, so information here is to take carefully.
What is known:
As Meamo has Debian as an upstream distribution, there have been lots of questions on what will be MeeGo's upstream, and why to drop the relation of Maemo with Debian. The answer is that MeeGo is an upstream distribution : the only upstream is the projects included into MeeGo such as : kernel.org, qt ...etc. MeeGo policy is to push all bug fixes to upstream projets.
Why not be Debian-based ? Having an upstream distribustion is both an advantage and a drawback. Avantage : you benefit from all the packaging done in the upstream distribution, thus there is a large choice of software easy to port. Drawback : you have to align the built infrastructure with the upstream. You are dependant on the release cycle and architecture choices of upstream.
The choice is to have no upstream for MeeGo. It makes it easier to tailor MeeGo for it's target devices, which is quite specific compared to Debian's one. Maybe the Debian rythm of updates would also be inadequate for MeeGo (smartphone OS are evolving at a very rapid rythm currently). The Moblin built tools would also not have been compatible with Debian as upstream. Nontheless MeeGo is Linux Standard Base (LSB) compliant and its specific component will probably be available in Debian at some point.
RPM and DEB in themselves are package format with quite equivalent feature. So the question to use one or the other is more related to the build tools you are willing to use. The choice for MeeGo is to use Moblin Tools and OpenSuse Built Services (OBS). The RPM choice is mainly a consequence of this.