The build infrastructure is not announced yet, so information here is speculative.
What is known:
As Maemo has had an old version of Debian as an upstream distribution, there have been lots of questions on what will be MeeGo's upstream, and why Maemo's relationship with Debian will be dropped. The answer is that MeeGo is an independent distribution : the only upstream is the projects included into MeeGo such as : kernel.org, X.org, Qt, etc. MeeGo policy is to push all bug fixes to upstream projects.
Why not be Debian-based ? Having an upstream distribution has advantages and disadvantages. Advantages : you benefit from all the packaging, QA and maintenance done in the upstream distribution, thus there is a large selection of available ported software. Disadvantages : you have to align your build infrastructure with your upstream. You are dependent 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 its target devices, which is quite specific compared to Debian's. Maybe the Debian rhythm of updates would also be inadequate for MeeGo (smartphone OSs are currently evolving at a very rapid pace, but there is more up-to-date Ubuntu and Debian is working on more frequent updates). The Moblin build tools would also not have been compatible with Debian as upstream (this is mostly nonsense, see Mer and OBS). Nonetheless MeeGo will be Linux Standard Base (LSB) compliant and its specific components will probably be available in Debian at some point.
RPM and DEB in themselves are package formats with roughly equivalent features. So the question to use one or the other is more related to the build tools you decide to use. The choice for MeeGo is to use Moblin Tools and openSUSE Build Services (OBS). The RPM choice is mainly a consequence of this (this is again nonsense, see Mer).
Evolving packaging guidelines...