Do you have simple repro steps on a vanilla Orchard install?
Hi, thx for your promptly response!
Now it works, and I can't reproduce it anymore..
At some point I've tried to delete my template alternates, and made some changes to Placement.info in the theme, and the content picker field started to show up again.
Then I moved my cshtmls back in place, and restored my custom Placement.info, and now it works as expected... I think it was only a temporary condition.
But if it happens again I'll post here or in the issue tracker.