Zen Cart v1.5.4 was released yesterday, primarily containing bugfixes and performance improvements over and above Zen Cart v1.5.3; this version (like v1.5.0) is PCI-compliant.
One change of note is the addition of an admin function: zen_record_admin_activity. This function is now widely used within the various Zen Cart admin modules, especially in the (ever popular) /YOUR_ADMIN/orders.php. I’ve developed a backward-compatibility script that any Zen Cart plugin author can use; this script enables authors to upgrade their admin-level core-file overwrites to use the Zen Cart v1.5.4 version without the need for Zen Cart version-specific instructions in their installations. This script (two files) is posted here on the Zen Cart forums.
At this time, all of my Zen Cart plugins (both free and commerical) have either been updated or verified to operate with Zen Cart v1.5.4!