Before getting too concerned about this latest Canonical move, how it's being promoted right now is to just complement apt/dpkg and to not replace the traditional Debian packages. This new package format and installer would just be for newly-distributed packages, namely third-party software on the Ubuntu Phone/Tablet and written against the Ubuntu SDK.
Colin Watson shared these initial plans on the Ubuntu-Devel list. Colin explained, "it'd just be second-system-effect on top of our current packaging system. So the scope of what I've been considering is purely leaf apps built on a fixed "base system", which in the case of the initial target of the Ubuntu phone/tablet work would be the run-time part of the Ubuntu SDK."
The proposed features of this new package installer/format would be no dependencies between applications (packages would only depend upon the base system and ship everything they need bundled within the package), each package would install to its own directory, the entire package format would be purely declarative, and the package format/building seems like it will revolve largely around Python and JSON.
There's still a lot of work ahead but they already have proof of concept code working with the current system written in Python and acceptable performance. Code will supposedly be published next week.
In Watson's mailing list post he did bring up "why not use an existing system out there?" to complement apt/dpkg, considering the vast selection of open-source package management systems... The answer is that they still might but that it wouldn't make much difference. The longer explanation appears to be they want the package format catered to their platform more than anything else with each package format already being rather designed around its own system.