Substrate is still a work in progress, but I am already running it in a small production site, the Seattle Urban Art aggregator
s3a. It is built on
Halo which is in turn built on top of Substrate.
There is a blog post discussing the usage of
Repose (a PHP ORM), Halo and Substrate together to build the s3a site on
Not Invented Here called
Tying It All Together. It goes a long ways towards showing Substrate used in a pretty complex application with a lot of dependency injection requirements.
There is also the
SampleCode on the Substrate Wiki if you are looking to see what Substrate can do on the simple side of things.
For anyone who has used
Spring Framework, these examples will hopefully look a little familiar.