2011/4/21 K. Peachey p858snake@gmail.com
http://www.mediawiki.org/wiki/Manual:$wgExternalLinkTarget can be done to effect all outbound links.
Here on the outdated page about it, gives a little bit of information about why people really dislike it when you do that:
http://www.mediawiki.org/w/index.php?title=Manual:Opening_external_links_in_...
Could this option be added to user preferences?
Alex