9 months ago, we deployed our site http://www.hamiltonmovingsolutions.com/ and it quickly obtained a PR4. Since that time it fell back to a PR0. Now I know that it needs links that we are working on but do you think the home page Tab Menu design might be considered "hidden text".
I really doubt that would be considered hidden text. Think of how many websites use similar techniques for some type of pop-up navigation menu. Perhaps it is some kind of duplicate content issue. I noticed that you do not have a redirect from non-www to the www version. Also the same page is accessible at http://www.hamiltonmovingsolutions.com/default.aspx Or perhaps it doesn't like those two w3.org links at the bottom. The positioning of those links could've trigger some kind of paid link filter.
No. In fact what you have is a canonical URI issue. Your server (I'm presuming it's IIS here) is serving not only four different versions of your home page, but you're also serving two different home page destinations. Here's what you need to do - and this is very important. Pick either .www or non-www and redirect the other to it. If you were using Apache, I'd give you the mod_rewrite code to do this, but IIS has its own way of doing things (note: there is a way to emulate mod_rewrite in IIS; if you buy the module, you'll be able to do that). Remove the default.aspx link in all of your pages and point it to the domain root.
I don't think it's the Tab Menu at all. Although it is quite annoying. You might wanna try what Dan suggested.