I've compared the expected behaviour from looking at wikipedia's cheat sheet at: http://en.wikipedia.org/wiki/Wikipedia:Cheatsheet and tried to create an external link with link text. This works fine, however if the external link is followed by an internal link, any space seperating this two types of link is removed by the wiki rending process. For example:
Wiki markup:
The quick http://brown.com red [[fox]] jumps
Resulting HTML:
<p> The quick<a href="http://brown.com"> red</a><a href="view.php?id=7&page=fox&gid=0&uid=0">fox</a> jumps </p>
But this should be:
<p> The quick<a href="http://brown.com">red</a> <a href="view.php?id=7&page=fox&gid=0&uid=0">fox</a> jumps </p>
Attached is an image of the rendered HTML. Note the lack of a space character between the "red" and "fox" links.
--Stuart