On the Symfony blog there's a recent post talking about the availability of the SF2 packages as standalone components available for separate downloads.

Each Symfony Component has been available as a standalone "package" for a very long time, but this is the first time I post something about this on this blog. That's because the way it was done was quite experimental... until recently. Why is it useful? Let's say you have a project that does not use Symfony, the full-stack framework, but you still want to rely on a few Symfony Components like YAML, Console, and Process for instance. In that case, instead of depending on the main symfony/symfony code, your project can just depend on these specific components.

An example of the composer.json configuration needed to grab individual components and define a "minimum stability" for the sources.

The new version has been online for the last week and the Components are now updated 1 or 2 minutes after some changes are pushed to the Symfony component. Unfortunately, we have had one force push during that time due to a bug in the Git subtree command (I haven't found the time to submit a bug report yet).