If a page has no interlanguage links then "Add links" is missing in MonoBook but present in Vector, Cologne Blue and Modern.
If there are interlanguage links then "Edit links" is correctly present in MonoBook. Example:
Considering the timing, this may be caused by 1.24wmf2.
The above was tested in Firefox, Opera, Safari.
In IE9 I'm missing "Add links" in all skins, but have "Edit links" in all skins.