Are you viewing it in IE 6 or 7? If so, in those browsers the site becomes a fixed width (slightly narrower) because of IE's lack of support of max-width and media queries. If you look at the bottom of the main Slate marketplace page you can see that I've noted that.

Customer is usng IE8 - Sorry Chris, I overlooked your "Statement" on the browsers.

I just checked on one of my dev computers that has Win7-IE9 and menu still wraps. So does Chrome unfortunately.

I know that font sizes can differ on PC/Mac and different browsers. That could be what's causing it. Can you post screenshots of what's happening.

Screen shots Firefox ok , IE 8, 9, Chrome menu line wraps

Yeah, it's just what I mentioned before. It's just how the browsers are rendering the text (size) slightly different and it's just enough to cause it to wrap. You can either shorten the names of the pages or decrease the space between menu items until it looks ok in all browsers.
You'll need to change the left/right padding on "nav ul li a" in the main.css file.

Thanks for the direction - I ended up decreasing the menu text font just a touch to make it fit.

All good!!

Great!