As a mediawiki and sharepoint user and admin, editing in place is one of the most required features but also a sign of not knowing what are the consequences. Most tipicaly, from a documentation point of view, edit in place completely trashes any kind of version management, rendering the whole platform useless. If users need to edit in place, use a file system, and forget about proper versioning.
O_o But aren't normal mediawiki articles edited 'in-place'? :)