From a quick check of the code:
All Moodle versions from 2.0 and above - all plugins (except for activity modules) expect version.php to contain $plugin->... details.
Moodle 2.0-2.4: activity modules must have $module->... in version.php
Moodle 2.5-2.9 (proposed): activity modules can have either $plugin->... or $module->... (but $plugin is the preferred version from 2.7 onwards)
Moodle 2.10: activity modules MUST have $plugin->...
I guess if you were really worried about keeping a version that is compatible with all Moodle versions you might be able to get away with putting both into your version.php file (but with 2 years between Moodle 2.5 and Moodle 2.10, and Moodle 2.4 being unsupported by the time Moodle 2.10 comes out, I probably wouldn't worry about it too much).