I just edited the tag wiki for mercurial on SO, SF, SU, and programmers. Since the tag covers the same program on all those sites, I copy-pasted the description from SO after merging in some details from the other sites.
In such a case, it would make a lot of sense if we could create a cross-site link between the tags. I realize that there are tags you don't want linked across different sites, so this should probably be done explicitly.
The link could work so that I can copy an existing description from another site. This would be just like using copy-paste to copy a description from one site to another, but easier.
A more advanced version would let me do the opposite: copy a description from one site to many other sites. Perhaps this should only be allowed if the description on the other sites is unchanged since it was copied the last time. That would mean that the "link" between sites is broken as soon as a site begins to customize the wiki for their particular purpose.