wiki 1.x (the one distributed as part of Moodle 1.x core), has one undocumented feature: interwiki links. And it has been there, anonymously, since the beginning of the module.
Recently we "discovered" (MDL-19460) it was causing some annoyances to pages named "Wiki: whatever" and others, causing the module to automatically redirect to other URLs (interwiki ones).
As long as the feature is an undocumented one (and unsupported) we have decided to disable it by default.
So starting next weekly build (June 17th 2009) those interwiki links won't be working anymore under Moodle 1.x.
In any case, as last resort... if somebody was using such feature it's possible to enable it again by defining:
$CFG->wiki_allow_interwiki = true;
in your config.php file.
Finally, be noted that it isn't really supported and, in fact, the interwiki links being provided are really outdated so, in order to get "modern" interwiki links your would need to hack the module. Definitively it's better to keep it disabled, IMO.
That's all, ciao