MediaWiki 1.5 alpha 2 includes a lot of bug fixes, feature merges,
Brion,
on testwiki which appears to run this version, I still see in [[Special:preferences]] subpage MISC the user option and a checkbox
"Show number of watching users"
The feature is described in http://bugzilla.wikipedia.org/show_bug.cgi?id=727 Number of watching users per page to be shown on recent changes view (OK)
I introduced this once with my patch, but it seems that the feature has been dropped and the removal of the option text has been simply forgotten.
In SpecialPreferences.php the line "$wgRCShowWatchingUsers ? 'shownumberswatching' : false," and the "global $wgRCShowWatchingUsers" should be removed.
T.