Okay, since I started this I might as well add the answer I got from the PyQt mailing list :

> The activated() signal is a "Qt 3 support member" of QAction:
> *http://doc.qt.nokia.com/latest/qaction-qt3.html
> This means that it is available if Qt has been built to contain code to
> support APIs in Qt 3, to help people port applications from Qt 3 to Qt 4.
> I guess the version of Qt for Maemo doesn't contain this code, so it won't
> work.

This thankfully solves the "mystery" and answers all my questions :

- most ready-built PyQt packages for desktops (Windows and probably Linux too) are compiled with the Qt3-compatibility options, so "activated" works,

- PyQt on Mameo is not, probably for space/performance reasons, so only "triggered" is recognized,

- eric4 is not plain wrong in proposing "activated" as a signal for QAction, but neither is it marked as deprecated in favour of "triggered".

Whew :-)