Structured data storage (as, for example, PHP sessions), on key / value storage server (memcached, redis, ...) or simply in flat file is an impoortant need.
This new extension is still considered as experimental (beta version).
Installation, on Fedora or Enterprise Linux with php 5.3
yum --enablerepo=remi install php-pecl-msgpack
on Enterprise Linux with php 5.4
yum --enablerepo=remi,remi-test install php-pecl-msgpack
So, you can test it now.
According to tests, feedback, and its interest, I will consider to propose this new package for the official repository.