I can speak fairly authoritatively on this subject, as the issue of Schengen and borders is a personal interest of mine.
What will happen in practice is that Schengen is suspended, but they will not control everyone. There will likely be a visible presence at key points (for instance, the escalator exit from the Schengen zone in Okęcie) - and that presence will likely be semi-permanent, but there's no way that everyone will be controlled. On the land borders, you'll almost certainly see areas set up for border control at the main crossings (Swiecko, Kudowa-Slone, etc) - but again, these won't be systematic and it's likely that they will pull people into areas specifically for the purpose rather than checking everyone.
This picture (although a bit small) shows it well -
It's highly unlikely that there will be even a visible control at the vast majority of border crossings. I'm going up to the border this weekend, so I'll check the situation on the ground and report back. These controls aren't uncommon - I've been stopped several times on the German-Polish border.
Are full border controls even legal for a Schengen member?
According to Schengen law, it's allowed to suspend Schengen for up to 30 days based on a risk to security - in practice, no-one (not even the Danes) have ever suspended Schengen completely to the point of outlawing border crossings anywhere except at approved points.