Based on my prior experience with the Samsung Captivate, I'm betting they aren't stock apps. Samsung seemed to be doing minor look changes, but major UI changes on 2.2, wouldn't surprise me to see them digging deeper with 2.3.

This is what the stock calendar looks like:
http://www.gsmarena.com/google_nexus_s-review-565p7.php

Messaging is here:
http://www.gsmarena.com/google_nexus_s-review-565p4.php