What's interesting is that we all know something like this needs to happen for the PHP community, but the community isn't stepping up and rallying around one. Maybe being sucked into the Symfony world will help it get traction, but I am not sure, only because history has not been on the side of progress of this issue in the PHP community.
PEARFarm hasn't caught on too much (though we use it extensively in-house).
Composer works almost identically to http://pearhub.org, and didn't catch on, either.
Beyond that, there have been more (see phark, etc).=
As for feedback on your idea, my concerns are: 1) Support for legacy PEAR packages doesn't seem to exist. There is a lot of code out there that people already depend on. 2) PHP 5.3 only, while it is the future, still knocks down the addressable market to maybe 1/4 of the PHP world. 3) Does your packaging system support installation of executables? 4) Is there a bundler-style solution? I need to be able to easily install all dependencies in a sandbox. Committing an enormous vendor/ dir is an absurd practice. 5) Does Composer allow the package to not include all files in the git repo?
That said, I would love for there to be a standard, widely-used, useful, easy & practical way to manifest dependencies and install them into a sandbox. I waste way too much time managing this on my own.