That's just a IE specific security issue - to prevent running malicious script.
IE has a habit of dumping anything after the 'about:' prompt in to the document as if like a document.write action. So a malicious scripter can ruin your patience.
Code:
Without that spacing between about and :
[url ]about:<body onload=%22while(true){alert('hi moron!')}%22></body>[/url]
will be converted to
<a href="about:<body onload=%22while(true){alert('hi moron!')}%22></body>">Click me</a>
and that is going to be a pain for the unwary clicker.
To see what I mean
Download the attached page(change the extension and open it in IE).