When posting a hyperlink to a URL, the link does not underline, and the text does not change colors, like a hyperlink should.
This is just how they did the forum skin - you can change the default rules of HTML in a CSS file. The links actually DO change color, albeit from grey to black, so if you have an LCD that doesn't do shades of grey well, it may not be noticeable.
I agree the no underline thing is bad, so I'm going to change that right after I post this. If people are still having trouble telling what's a link and what isn't, I'll also make links red or blue.