I posted this problem to the support area for the plugin at issue but have heard nothing after 3 weeks. Figured I'd try here in case the problem can be solved from this end.
Issue is with p2-check-in. The widget had been working properly, showing who is checked in to the site. It included a button in the header area that allowed users to check in and check out. When P2 was upgraded to 1.4.3, the button in the header area disappeared. Those users checked in still show up in the side bar area as before, but of course, can't be shown as 'checked out' because the button is non-existent and can't be pushed!
This is the plugin: http://wordpress.org/plugins/p2-check-in/
I could abandon it completely, but it was serving a useful purpose for me...
Any easy fix, or can we determine whether the problem should be attributed to the P2 or the plugin?