> <http://www.osha.gov/pls/oshaweb/owaredirect.html?>>;
p_url=http://www.httrack.com
> the url is clearly okay so I'm thinking it's just an 
> unhandled case within httrack.

No - it is handled, but you did not tell httrack to follow 
the link.

In this example, starting from 
<http://www.osha.gov/pls/oshaweb/owaredirect.html?p_url=http://www.httrack.com>

you'll have also to define the following Scan rule: 
(Options/Scan rules)
+http://www.httrack.com/*