For all the Web Parts on my SP 2010 team site wiki page, the "Hidden" option is greyed out (under Edit Web Part > Layout).
How can I activate this option?
I ran into the same issue a while ago. As a temporary workaround I exported the WebPart into a DWP file and edited it with a text editor (I prefer Notepad++). Once done I imported it back and added it to the page.