There are hints that it's free only for a preview period. From http://www.redhat.com/products/cloud-computing/openshift/, under the "How To Buy" tab, it calls it a "free developer preview" and says, "OpenShift is currently in developer preview and free to use. Stay tuned—pricing is coming soon."

But what's the catch?

It's Free Software, you can run it yourself or switch providers at will. The catch may be in the lack of features compared to a proprietary option (like Heroku)

From their faq page: To get more information on the pricing and timing of a paid version of OpenShift please send an email to [email protected]

Yeah, is it free as in beer? For life? No matter how much it has to scale to?

If it is, I would like to understand the reasoning behind that? Is it to gain traction and visibility for Red Hate as a PaaS? And then...

It is in open beta and has big warning labels to not use in production.

The free tier is limited in resources; When you want more, you'll be expected to pay for more (once they begin to offer paid plans).